ADT7461A Datasheet by ON Semiconductor

ADT7461 A The ADT7461A is a dual—channel dig undertemperalurc/overlemperalure alarm, inlen thermal manugemenl systems. It is pin— and reg ADMll)32 and ADT7461. A feature of 1 re nce cancellalion, where up [u 1.5 kQ series with the lemperulure muniloring di cancelled frum Ihe iempenuure result, all ADT7461A ha. a Configurable ALERT switchable temperature measurement range The AD17461A can measure the lem diode accurate Io t1°C and the ambient The lemperalure measurement range cumpnlible with [he ADM1032, but it measurement range uf —64°C In +191°C. The ADT7461A communicates over cumpulible with system management h default SMBus address of the ADT7461A available with an SMBus address of l)x4D AD17461A t used on Ihe same SMBus. An ALiERT eutpu the on—chi out of mnge. The TERM is h com on/eer control of a Co ALERT as a second THERM Up to Two Overlemperalure Fail—Safe Til-IERM 0N Semimnrluetor® Q

October, 2009 − Rev. 5

1Publication Order Number:

ADT7461A/D

ADT7461A

+15C Temperature Monitor

with Series Resistance

Cancellation

The ADT7461A is a dual−channel digital thermometer and

undertemperature/overtemperature alarm, intended for use in PCs and

thermal management systems. It is pin− and register−compatible with the

ADM1032 and ADT7461. A feature of the ADT7461A is series

resistance cancellation, where up to 1.5 kW (typical) of resistance in

series with the temperature monitoring diode can be automatically

cancelled from the temperature result, allowing noise filtering. The

ADT7461A has a configurable ALERT output and an extended,

switchable temperature measurement range.

The ADT7461A can measure the temperature of a remote thermal

diode accurate to ±1°C and the ambient temperature accurate to ±3°C.

The temperature measurement range defaults to 0°C to +127°C,

compatible with the ADM1032, but it can be switched to a wider

measurement range of −64°C to +191°C.

The ADT7461A communicates over a 2−wire serial interface,

compatible with system management bus (SMBus) standards. The

default SMBus address of the ADT7461A is 0x4C. An ADT7461A−2 is

available with an SMBus address of 0x4D. This is useful if more than one

ADT7461A is used on the same SMBus.

An ALERT output signals when the on−chip or remote temperature is

out of range. The THERM output is a comparator output that allows

on/off control of a cooling fan. The ALERT output can be reconfigured

as a second THERM output, if required.

Features

•On−Chip and Remote Temperature Sensor

•0.25°C Resolution/1°C Accuracy on Remote Channel

•1°C Resolution/1°C Accuracy on Local Channel

•Automatically Cancels Up to 1.5 kW (Typ) of Resistance in Series

with Remote Diode to Allow Noise Filtering

•Extended, Switchable Temperature Measurement Range

0°C to +127°C (Default) or –64°C to +191°C

•Pin− and Register−Compatible with ADM1032 and ADT7461

•2−Wire SMBus Serial Interface with SMBus Alert Support

•Programmable Over/Undertemperature Limits

•Offset Registers for System Calibration

•Up to Two Overtemperature Fail−Safe THERM Outputs

•Small 8−Lead MSOP

•240 mA Operating Current, 5 mA Standby Current

•These are Pb−Free Devices

Applications

•Desktop and Notebook Computers

•Industrial Controllers

•Smart Batteries

•Automotive

•Embedded Systems

•Burn−In Applications

•Instrumentation

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PIN ASSIGNMENT

VDD

D+

D–

GND

ALERT/THERM2

THERM

(Top View)

SCLK

SDATA

See detailed ordering and shipping information in the package

dimensions section on page 18 of this data sheet.

ORDERING INFORMATION

MSOP−8

CASE 846AB

T1x = Refer to Order Info Table

R = Assembly Location

Y = Year

W = Work Week

G= Pb−Free Package

(Note: Microdot may be in either location)

T1x

RYWG

Tanninzmz Tm SCLK‘ SDATA‘ FEERT TFI'EFWI Input Current SDATA, TFI'EFWI

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Figure 1. Functional Block Diagram

A−TO−D

CONVERTER

RUN/STANDBYBUSY

REMOTE OFFSET

LIMIT

COMPARATOR

STATUS REGISTER

INTERRUPT

MASKING

REMOTE TEMPERATURE

VALUE REGISTER

LOCAL TEMPERATURE

VALUE REGISTER

ON−CHIP

TEMPERATURE

SENSOR

ANALOG

MUX

EXTERNAL DIODE OPEN−CIRCUIT

SMBus INTERFACEADT7461A

157

VDD GND SDATA SCLK

THERM

D+

D–

ALERT/THERM2

DIGITAL MUX

CONFIGURATION

REGISTERS

EXTERNAL THERM LIMIT

REGISTERS

LOCAL THERM LIMIT

REGISTERS

REMOTE TEMPERATURE

HIGH−LIMIT REGISTER

REMOTE TEMPERATURE

LOW−LIMIT REGISTER

LOCAL TEMPERATURE

HIGH−LIMIT REGISTER

LOCAL TEMPERATURE

LOW−LIMIT REGISTER

CONVERSION RATE

ADDRESS POINTER

DIGITAL MUX

ABSOLUTE MAXIMUM RATINGS

Parameter Rating Unit

Positive Supply Voltage (VDD) to GND −0.3, +3.6 V

D+ −0.3 to VDD + 0.3 V

D− to GND −0.3 to +0.6 V

SCLK, SDATA, ALERT, THERM −0.3 to +3.6 V

Input Current, SDATA, THERM −1, +50 mA

Input Current, D−±1 mA

ESD Rating, All Pins (Human Body Model) 1500 V

Maximum Junction Temperature (TJ Max) 150 °C

Storage Temperature Range −65 to +150 °C

Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the

Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect

device reliability.

NOTE: This device is ESD sensitive. Use standard ESD precautions when handling.

THERMAL CHARACTERISTICS

Package Type qJA qJC Unit

8−Lead MSOP 142 43.74 °C/W

HEW/mm Open-Dram Logic 0mm Used as Interrupl or SMEus W Second TFI'EH'M

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PIN ASSIGNMENT

Pin No. Mnemonic Description

1 VDD Positive Supply, 3.0 V to 3.6 V.

2 D+ Positive Connection to Remote Temperature Sensor.

3 D−Negative Connection to Remote Temperature Sensor.

4 THERM Open−Drain Output. Can be used to turn a fan on/off or throttle a CPU clock in the event of an

overtemperature condition. Requires pullup resistor.

5 GND Supply Ground Connection.

6 ALERT/THERM2 Open−Drain Logic Output Used as Interrupt or SMBus ALERT. This can also be configured as a

second THERM output. Requires pullup resistor.

7 SDATA Logic Input/Output, SMBus Serial Data. Open−Drain Output. Requires pullup resistor.

8SCLK Logic Input, SMBus Serial Clock. Requires pullup resistor.

SMBus TIMING SPECIFICATIONS (Note 1)

Parameter Limit at TMIN and TMAX Unit Description

fSCLK 400 kHz max −

tLOW 1.3 ms min Clock low period, between 10% points.

tHIGH 0.6 ms min Clock high period, between 90% points.

tR300 ns max Clock/data rise time.

tF300 ns max Clock/data fall time.

tSU; STA 600 ns min Start condition setup time.

tHD; STA (Note 2) 600 ns min Start condition hold time.

tSU; DAT (Note 3) 100 ns min Data setup time.

tSU; STO (Note 4) 600 ns min Stop condition setup time.

tBUF 1.3 ms min Bus free time between stop and start conditions.

1. Guaranteed by design, but not production tested.

2. Time from 10% of SDATA to 90% of SCLK.

3. Time for 10% or 90% of SDATA to 10% of SCLK.

4. Time for 90% of SCLK to 10% of SDATA.

Figure 2. Serial Bus Timing

SCLK

SDATA

tRtF

tLOW

tHD;DAT

tHD;STA tHIGH

tSU;DAT

STOP START STOPSTART

tSU;STA tSU;STO

tHD;STA

tBUF

Open-Dram Dlgltal Outputs mm, mm

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ELECTRICAL CHARACTERISTICS TA = −40°C to +125°C, VDD = 3.0 V to 3.6 V, unless otherwise noted.

Parameter Conditions Min Typ Max Unit

Power Supply

Supply Voltage, VDD 3.0 3.30 3.6 V

Average Operating Supply Current, IDD 0.0625 Conversions/Sec Rate (Note 1)

Standby mode

−240

5.0

350

Undervoltage Lockout Threshold VDD input, disables ADC, rising edge −2.55 −V

Power−On−Reset Threshold 1.0 −2.5 V

Temperature−To−Digital Converter

Local Sensor Accuracy 0°C ≤ TA ≤ +70°C

0°C ≤ TA ≤ +85°C

−40°C ≤ TA ≤ +100°C

− − ±1.0

±1.5

±2.5

°C

Resolution −1.0 −°C

Remote Diode Sensor Accuracy 0°C ≤ TA ≤ +70°C, −55°C ≤ TD (Note 2) ≤ +150°C

0°C ≤ TA ≤ +85°C, −55°C ≤ TD (Note 2) ≤ +150°C

−40°C ≤ TA ≤ +100°C, −55°C ≤ TD (Note 2) ≤ +150°C

− − ±1.0

±1.5

±2.5

°C

Resolution −0.25 −°C

Remote Sensor Source Current High level (Note 3)

Middle level (Note 3)

Low level (Note 3)

−220

13.5

−mA

Conversion Time From stop bit to conversion complete, one−shot

mode with averaging switched on

−40 52 ms

One−shot mode with averaging off (that is, conversion

rate = 16−, 32−, or 64−conversions per second)

−6.0 8.0 ms

Maximum Series Resistance Cancelled Resistance split evenly on both the D+ and D– inputs −1.5 −kW

Open−Drain Digital Outputs (THERM, ALERT/THERM2)

Output Low Voltage, VOL IOUT = −6.0 mA − − 0.4 V

High Level Output Leakage Current, IOH VOUT = VDD −0.1 1.0 mA

SMBus Interface (Note 3 and 4)

Logic Input High Voltage, VIH SCLK, SDATA 3.0 V ≤ VDD ≤ 3.6 V 2.1 − − V

Logic Input Low Voltage, VIL SCLK, SDATA 3.0 V ≤ VDD ≤ 3.6 V − − 0.8 V

Hysteresis −500 −mV

SDA Output Low Voltage, VOL − − 0.4 mA

Logic Input Current, IIH, IIL −1.0 −+1.0 mA

SMBus Input Capacitance, SCLK, SDATA −5.0 −pF

SMBus Clock Frequency − − 400 kHz

SMBus Timeout (Note 5) User programmable −25 64 ms

SCLK Falling Edge to SDATA Valid Time Master clocking in data − − 1.0 ms

1. See Table 5 for information on other conversion rates.

2. Guaranteed by characterization, but not production tested.

3. Guaranteed by design, but not production tested.

4. See SMBus Timing Specifications section for more information.

5. Disabled by default. Detailed procedures to enable it are in the Serial Bus Interface section of the datasheet.

// / :: VJ \

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TYPICAL CHARACTERISTICS

Figure 3. Local Temperature Error vs. Temperature Figure 4. Remote Temperature Error vs. Actual

Temperature

Figure 5. Temperature Error vs. D+/D− Leakage

Resistance

Figure 6. Temperature Error vs. D+/D− Capacitance

Figure 7. Operating Supply Current vs.

Conversion Rate

Figure 8. Operating Supply Current vs. Voltage

3.5

–1.0

–50 150

TEMPERATURE (5C)

TEMPERATURE ERROR (5C)

3.0

2.5

2.0

1.5

1.0

0.5

050100

DEV 1

DEV 2

DEV 3

DEV 4

DEV 5

DEV 6

DEV 7

DEV 8

DEV 9

DEV 10

DEV 11

DEV 12

DEV 13

DEV 14

DEV 15

DEV 16

MEAN

HIGH 4

LOW 4

–0.5

3.5

–1.0

–50 150

TEMPERATURE ERROR (5C)

3.0

2.5

2.0

1.5

1.0

0.5

0 50 100

TEMPERATURE (5C)

DEV 1

DEV 2

DEV 3

DEV 4

DEV 5

DEV 6

DEV 7

DEV 8

DEV 9

DEV 10

DEV 11

DEV 12

DEV 13

DEV 14

DEV 15

DEV 16

HIGH 4

LOW 4

–0.5

–25

1100

LEAKAGE RESISTANCE (MΩ)

TEMPERATURE ERROR (5C)

–10

–15

–20

D+ TO GND

D+ TO VCC

–5

–18

025

CAPACITANCE (nF)

TEMPERATURE ERROR (5C)

–2

–4

–6

–12

5101520

DEV 3

DEV 2

DEV 4

–8

–10

–16

–14

1000

0.01 100

CONVERSION RATE (Hz)

IDD (mA)

900

800

700

600

500

400

300

200

100

0.1 1 10

DEV 4BC

DEV 3BC

DEV 2BC

422

408

3.0 3.6

VDD (V)

IDD (mA)

420

418

416

414

412

410

3.1 3.2 3.3 3.4 3.5

DEV 3BC

DEV 2BC

DEV 4BC

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TYPICAL CHARACTERISTICS

Figure 9. Standby Supply Current vs. Voltage Figure 10. Standby Supply Current vs. Clock

Frequency

Figure 11. Temperature Error vs. Common−Mode

Noise Frequency

Figure 12. Temperature Error vs.

Differential−Mode Noise Frequency

Figure 13. Temperature Error vs. Series Resistance

4.4

3.0

3.0 3.6

VDD (V)

IDD (mA)

4.2

4.0

3.8

3.6

3.4

3.2

3.1 3.2 3.3 3.4 3.5

DEV 3

DEV 2

DEV 4

1 1000

FSCL (kHz)

ISTBY (mA)

10 100

DEV 2BC

DEV 3BC

DEV 4BC

100 200 300 400 500 600

NOISE FREQUENCY (MHz)

TEMPERATURE ERROR (5C)

20mV

100mV

50mV

–10

100 200 300 400 500 600

NOISE FREQUENCY (MHz)

TEMPERATURE ERROR (5C)

20mV

100mV

50mV

0 2000

SERIES RESISTANCE ( )

TEMPERATURE ERROR (5C)

500 1000 1500

wilh the cmrcsponding high, low. and THERM ALERT AL RT fauh i.~ dc‘cc Ld. Exceeding Ihc m ,~ the m ompul m asscn low. The m can be reprogrammed as a 5 00nd m ALERT ALERT and THERMZ

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Differences between the ADT7461A and the ADT7461

Although the ADT7461A is pin− and register−compatible

with the ADT7461, there are some specification differences

between the two devices. A summary of these differences is

shown below in Table 1.

Table 1. Differences Between the ADT7461A

and the ADT7461

Specification ADT7461A ADT7461 Unit

Supply Voltage 3.0 to 3.6 3.0 to 5.5 V

Maximum Local Sensor

Accuracy

1.0 3.0 °C

Maximum Series

Resistance Cancellation

1.5 3.0 kW

Average Operating

Supply Current

16 Conversions/Sec

Standby Mode

240

5.0

170

5.5

Max Conversion Time

One Shot, Averaging On

One Shot, Averaging Off

8.0

114.6

12.56

Remote Sensor Current

Levels

High

Mid

Low

220

13.5

6.0

Theory of Operation

The ADT7461A is a local and remote temperature sensor

and over/under temperature alarm, with the added ability to

automatically cancel the effect of 1.5 kW (typical) of

resistance in series with the temperature monitoring diode.

When the ADT7461A is operating normally, the on−board

ADC operates in a free running mode. The analog input

multiplexer alternately selects either the on−chip

temperature sensor to measure its local temperature or the

remote temperature sensor. The ADC digitizes these signals

and the results are stored in the local and remote temperature

value registers.

The local and remote measurement results are compared

with the corresponding high, low, and THERM temperature

limits, stored in eight on−chip registers. Out−of−limit

comparisons generate flags that are stored in the status

the low temperature limit causes the ALERT output to

assert. The ALERT output also asserts if an external diode

fault is detected. Exceeding the THERM temperature limits

causes the THERM output to assert low. The ALERT output

can be reprogrammed as a second THERM output.

The limit registers are programmed and the device

controlled and configured via the serial SMBus. The

contents of any register are also read back via the SMBus.

Control and configuration functions consist of switching

the device between normal operation and standby mode,

selecting the temperature measurement range, masking or

enabling the ALERT output, switching Pin 6 between

ALERT and THERM2, and selecting the conversion rate.

Series Resistance Cancellation

Parasitic resistance to the D+ and D− inputs to the

ADT7461A, seen in series with the remote diode, is caused

by a variety of factors, including PCB track resistance and

track length. This series resistance appears as a temperature

offset in the remote sensor’s temperature measurement. This

error typically causes a 0.5°C offset per ohm of parasitic

resistance in series with the remote diode.

The ADT7461A automatically cancels the effect of this

series resistance on the temperature reading, giving a more

accurate result, without the need for user characterization of

this resistance. The ADT7461A is designed to automatically

cancel typically up to 1.5 kW of resistance. By using an

advanced temperature measurement method, this process is

transparent to the user. This feature permits resistances to be

added to the sensor path to produce a filter, allowing the part

to be used in noisy environments. See the section on Noise

Filtering for more details.

Temperature Measurement Method

A simple method of measuring temperature is to exploit

the negative temperature coefficient of a diode, measuring

the base emitter voltage (VBE) of a transistor operated at

constant current. However, this technique requires

calibration to null the effect of the absolute value of VBE,

which varies from device to device.

The technique used in the ADT7461A measures the

change in VBE when the device operates at three different

currents. Previous devices used only two operating currents,

but it is the use of a third current that allows automatic

cancellation of resistances in series with the external

temperature sensor.

Figure 14 shows the input signal conditioning used to

measure the output of an external temperature sensor. This

figure shows the external sensor as a substrate transistor, but

it can equally be a discrete transistor. If a discrete transistor

is used, the collector is not grounded but is linked to the base.

To prevent ground noise interfering with the measurement,

the more negative terminal of the sensor is not referenced to

ground, but is biased above ground by an internal diode at

the D− input. C1 may be added as a noise filter (a

recommended maximum value of 1000 pF). However, a

better option in noisy environments is to add a filter, as

described in the Noise Filtering section. See the Layout

Considerations section for more information on C1.

To measure DVBE, the operating current through the

sensor is switched among three related currents. As shown

in Figure 14, N1 x I and N2 x I are different multiples of the

current, I. The currents through the temperature diode are

switched between I and N1 x I, giving DVBE1; and then

between I and N2 x I, giving DVBE2. The temperature is then

calculated using the two DVBE measurements. This method

also cancels the effect of any series resistance on the

temperature measurement.

The resulting DVBE waveforms are passed through a

65 kHz low−pass filter to remove noise and then to a

we} is;

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chopper−stabilized amplifier. This amplifies and rectifies

the waveform to produce a dc voltage proportional to DVBE.

The ADC digitizes this voltage producing a temperature

measurement. To reduce the effects of noise, digital filtering

is performed by averaging the results of 16 measurement

cycles for low conversion rates. At rates of 16−, 32−, and

64−conversions/second, no digital averaging occurs.

Signal conditioning and measurement of the internal

temperature sensor are performed in the same manner.

Figure 14. Input Signal Conditioning

VDD

IBIASN2 y II

BIAS

DIODE

D+

D–

C11

1CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS.

REMOTE

SENSING

TRANSISTOR fC = 65kHz

VOUT+

VOUT–

TO ADC

N1 y I

C1 = 1000pF MAX.

Temperature Measurement Results

The results of the local and remote temperature

measurements are stored in the local and remote temperature

value registers and compared with limits programmed into

the local and remote high and low limit registers.

The local temperature value is in Register 0x00 and has a

resolution of 1°C. The external temperature value is stored

in two registers, with the upper byte in Register 0x01 and the

lower byte in Register 0x10. Only the two MSBs in the

external temperature low byte are used giving the external

temperature measurement a resolution of 0.25°C. Table 2

lists the data format for the external temperature low byte.

Table 2. Extended Temperature Resolution

(Remote Temperature Low Byte

Extended Resolution Remote Temperature Low Byte

0.00°C0 000 0000

0.25°C0 100 0000

0.50°C1 000 0000

0.75°C1 100 0000

When reading the full external temperature value, read the

LSB first. This causes the MSB to be locked (that is, the

ADC does not write to it) until it is read. This feature ensures

that the results read back from the two registers come from

the same measurement.

Temperature Measurement Range

The temperature measurement range for both internal and

external measurements is, by default, 0°C to +127°C.

However, the ADT7461A can be operated using an

extended temperature range. The extended measurement

range is −64°C to +191°C. Therefore, the ADT7461A can be

used to measure the full temperature range of an external

diode, from −55°C to +150°C.

The extended temperature range is selected by setting Bit

2 of the configuration register to 1. The temperature range

is 0°C to 127°C when Bit 2 equals 0. A valid result is

available in the next measurement cycle after changing the

temperature range.

In extended temperature mode, the upper and lower

temperature that can be measured by the ADT7461A is

limited by the remote diode selection. The temperature

registers can have values from −64°C to +191°C. However,

most temperature sensing diodes have a maximum

temperature range of −55°C to +150°C. Above +150°C, they

may lose their semiconductor characteristics and

approximate conductors instead. This results in a diode

short. In this case, a read of the temperature result register

gives the last good temperature measurement. Therefore,

the temperature measurement on the external channel may

not be accurate for temperatures that are outside the

operating range of the remote sensor.

It should be noted that although both local and remote

temperature measurements can be made while the part is in

extended temperature mode, the ADT7461A itself should

not be exposed to temperatures greater than those specified

in the absolute maximum ratings section. Further, the device

is only guaranteed to operate as specified at ambient

temperatures from −40°C to +120°C.

Temperature Data Format

The ADT7461A has two temperature data formats. When

the temperature measurement range is from 0°C to 127°C

(default), the temperature data format for both internal and

external temperature results is binary. When the measurement

range is in extended mode, an offset binary data format is used

for both internal and external results. Temperature values are

offset by 64°C in the offset binary data format. Examples of

temperatures in both data formats are shown in Table 3.

m umpm. If Bi! 7 is 1), mm W . This applies only if Pin a is configured m m m 0 ﬁle ADT7461A via the SMBus‘ The ALERT and THERM THERM ALERT m m m acﬁvc when Pin 6 i. ‘onﬁgumd as an m o is set up as a m A0 32 22 FEERT/ THEM Ao

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Table 3. Temperature Data Format

(Temperature High Byte)

Temperature Binary Offset Binary (Note 1)

–55°C0 000 0000

(Note 2)

0 000 1001

0°C0 000 0000 0 100 0000

+1°C0 000 0001 0 100 0001

+10°C0 000 1010 0 100 1010

+25°C0 001 1001 0 101 1001

+50°C0 011 0010 0 111 0010

+75°C0 100 1011 1 000 1011

+100°C0 110 0100 1 010 0100

+125°C0 111 1101 1 011 1101

+127°C0 111 1111 1 011 1111

+150°C0 111 1111

(Note 3)

1 101 0110

1. Offset binary scale temperature values are offset by 64°C.

2. Binary scale temperature measurement returns 0°C for all

temperatures < 0°C.

3. Binary scale temperature measurement returns 127°C for all

temperatures > 127°C.

The user can switch between measurement ranges at any

time. Switching the range likewise switches the data format.

The next temperature result following the switching is

reported back to the register in the new format. However, the

contents of the limit registers do not change. It is up to the

user to ensure that when the data format changes, the limit

registers are reprogrammed as necessary. More information

on this is found in the Limit Registers section.

ADT7461A Registers

The ADT7461A contains 22, 8−bit registers in total.

These registers store the results of remote and local

temperature measurements, high and low temperature

limits, and configure and control the device. See the Address

Pointer Register section through the Consecutive ALERT

the ADT7461A registers. Additional details are shown in

Table 4 through Table 8. The entire register map is available

in Table 9.

Address Pointer Register

The address pointer register itself does not have, nor does

it require, an address because the first byte of every write

operation is automatically written to this register. The data

in this first byte always contains the address of another

pointer register. It is to this register address that the second

byte of a write operation is written, or to which a subsequent

read operation is performed.

The power−on default value of the address pointer register

is 0x00. Therefore, if a read operation is performed

immediately after power−on, without first writing to the

address pointer, the value of the local temperature is returned

because its register address is 0x00.

Temperature Value Registers

The ADT7461A has three registers to store the results of

local and remote temperature measurements. These

registers can only be written to by the ADC and can be read

by the user over the SMBus. The local temperature value

The external temperature value high byte register is at

Address 0x01, with the low byte register at Address 0x10.

The power−on default for all three registers is 0x00.

Configuration Register

The configuration register is Address 0x03 at read and

Address 0x09 at write. Its power−on default is 0x00. Only

four bits of the configuration register are used. Bit 0, Bit 1,

Bit 3, and Bit 4 are reserved; the user does not write to them.

Bit 7 of the configuration register masks the ALERT

output. If Bit 7 is 0, the ALERT output is enabled. This is the

power−on default. If Bit 7 is set to 1, the ALERT output is

disabled. This applies only if Pin 6 is configured as ALERT.

If Pin 6 is configured as THERM2, then the value of Bit 7

has no effect.

If Bit 6 is set to 0, which is power−on default, the device

is in operating mode with ADC converting. If Bit 6 is set to

1, the device is in standby mode and the ADC does not

convert. The SMBus does, however, remain active in

standby mode; therefore, values can be read from or written

to the ADT7461A via the SMBus. The ALERT and THERM

outputs are also active in standby mode. Changes made to

the registers in standby mode that affect the THERM or

ALERT outputs cause these signals to be updated.

Bit 5 determines the configuration of Pin 6 on the

ADT7461A. If Bit 5 is 0 (default), then Pin 6 is configured

as an ALERT output. If Bit 5 is 1, then Pin 6 is configured

as a THERM2 output. Bit 7, the ALERT mask bit, is only

active when Pin 6 is configured as an ALERT output. If Pin

6 is set up as a THERM2 output, then Bit 7 has no effect.

Bit 2 sets the temperature measurement range. If Bit 2 is

0 (default value), the temperature measurement range is set

between 0°C to +127°C. Setting Bit 2 to 1 sets the

measurement range to the extended temperature range

(−64°C to +191°C).

Table 4. Configuration Register Bit Assignments

Bit Name Function Power−On

Default

7MASK1 0 = ALERT Enabled

1 = ALERT Masked

6 RUN/STOP 0 = Run

1 = Standby

5 ALERT/

THERM2

0 = ALERT

1 = THERM2

4, 3 Reserved 0

2 Temperature

Range Select

0 = 0°C to 127°C

1 = Extended range

1, 0 Reserved 0

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Conversion Rate Register

The conversion rate register is Address 0x04 at read and

Address 0x0A at write. The lowest four bits of this register

are used to program the conversion rate by dividing the

internal oscillator clock by 1, 2, 4, 8, 16, 32, 64, 128, 256,

512, or 1024 to give conversion times from 15.5 ms (Code

0x0A) to 16 seconds (Code 0x00). For example, a

conversion rate of eight conversions per second means that

beginning at 125 ms intervals, the device performs a

conversion on the internal and the external temperature

channels.

The conversion rate register can be written to and read

back over the SMBus. The higher four bits of this register are

unused and must be set to 0. The default value of this register

is 0x08, giving a rate of 16 conversions per second. Use of

slower conversion times greatly reduces the device power

consumption.

Table 5. Conversion Rate Register Codes

Code Conversion/Second Time (Seconds)

0x00 0.0625 16

0x01 0.125 8

0x02 0.25 4

0x03 0.5 2

0x04 1 1

0x05 2 500 m

0x06 4 250 m

0x07 8 125 m

0x08 16 62.5 m

0x09 32 31.25 m

0x0A 64 15.5 m

0x0B to 0xFF Reserved

Limit Registers

The ADT7461A has eight limit registers: high, low, and

THERM temperature limits for both local and remote

temperature measurements. The remote temperature high

and low limits span two registers each, to contain an upper

and lower byte for each limit. There is also a THERM

hysteresis register. All limit registers can be written to, and

read back over, the SMBus. See Table 9 for details of the

limit register addresses and their power−on default values.

When Pin 6 is configured as an ALERT output, the high

limit registers perform a > comparison, while the low limit

registers perform a ≤ comparison. For example, if the high

limit register is programmed with 80°C, then measuring

81°C results in an out−of−limit condition, setting a flag in

the status register. If the low limit register is programmed

with 0°C, measuring 0°C or lower results in an out−of−limit

condition.

Exceeding either the local or remote THERM limit asserts

THERM low. When Pin 6 is configured as THERM2,

exceeding either the local or remote high limit asserts

THERM2 low. A default hysteresis value of 10°C is

provided that applies to both THERM channels. This

hysteresis value can be reprogrammed to any value after

powerup (Register Address 0x21).

It is important to remember that the temperature limits

data format is the same as the temperature measurement data

format. Therefore, if the temperature measurement uses

default binary, then the temperature limits also use the

binary scale. If the temperature measurement scale is

switched, however, the temperature limits do not

automatically switch. The user must reprogram the limit

registers to the desired value in the correct data format. For

example, if the remote low limit is set at 10°C with the

default binary scale, the limit register value is 0000 1010b.

If the scale is switched to offset binary, the value in the low

temperature limit register needs to be reprogrammed to

0100 1010b.

Status Register

The status register is a read−only register at Address 0x02.

It contains status information for the ADT7461A.

When Bit 7 of the status register is high, it indicates that the

ADC is busy converting. The other bits in this register flag the

out−of−limit temperature measurements (Bit 6 to Bit 3, and

Bit 1 to Bit 0) and the remote sensor open circuit (Bit 2).

If Pin 6 is configured as an ALERT output, the following

applies: If the local temperature measurement exceeds its

limits, Bit 6 (high limit) or Bit 5 (low limit) of the status

temperature measurement exceeds its limits, then Bit 4 (high

limit) or Bit 3 (low limit) asserts. Bit 2 asserts to flag an open

circuit condition on the remote sensor. These five flags are

NOR’ed together, so if any of them is high, the ALERT

interrupt latch is set and the ALERT output goes low.

Reading the status register clears the five flags, Bit 6 to

Bit 2, provided the error conditions causing the flags to be

set have gone away. A flag bit can be reset only if the

corresponding value register contains an in−limit

measurement or if the sensor is good.

The ALERT interrupt latch is not reset by reading the

status register. It resets when the ALERT output has been

serviced by the master reading the device address, provided

the error condition has gone away and the status register flag

bits are reset.

When Flag 1 and/or Flag 0 are set, the THERM output

goes low to indicate that the temperature measurements are

outside the programmed limits. The THERM output does

not need to be reset, unlike the ALERT output. Once the

measurements are within the limits, the corresponding status

goes high. The user may add hysteresis by programming

temperature falls to limit value minus the hysteresis value.

When Pin 6 is configured as THERM2, only the high

temperature limits are relevant. If Flag 6 and/or Flag 4 are

set, the THERM2 output goes low to indicate that the

temperature measurements are outside the programmed

limits. Flag 5 and Flag 3 have no effect on THERM2. The

behavior of THERM2 is otherwise the same as THERM.

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Table 6. Status Register Bit Assignments

Bit Name Function

7 BUSY 1 when ADC is converting

6 LHIGH

(Note 1)

1 when local high temperature limit is

tripped

5 LLOW

(Note 1)

1 when local low temperature limit is

tripped

4 RHIGH

(Note 1)

1 when remote high temperature limit is

tripped

3 RLOW

(Note 1)

1 when remote low temperature limit is

tripped

2 OPEN

(Note 1)

1 when remote sensor is an open circuit

1 RTHRM 1 when remote THERM limit is tripped

0 LTHRM 1 when local THERM limit is tripped

1. These flags stay high until the status register is read or they

are reset by POR unless Pin 6 is configured as THERM2.

Then, only Bit 2 remains high until the status register is read

or is reset by POR.

Offset Register

Offset errors can be introduced into the remote

temperature measurement by clock noise or when the

thermal diode is located away from the hot spot. To achieve

the specified accuracy on this channel, these offsets must be

removed.

The offset value is stored as a 10−bit, twos complement

value in Register 0x11 (high byte) and Register 0x12 (low

byte, left justified). Only the upper two bits of Register 0x12

are used. The MSB of Register 0x11 is the sign bit. The

minimum, programmable offset is −128°C, and the

maximum is +127.75°C. The value in the offset register is

added to, or subtracted from, the measured value of the

remote temperature.

The offset register powers up with a default value of 0°C

and has no effect unless the user writes a different value to it.

Table 7. Sample Offset Register Codes

Offset Value 0x11 0x12

−128°C1000 0000 00 00 0000

−4°C1111 1100 00 00 0000

−1°C1111 1111 00 000000

−0.25°C1111 1111 10 00 0000

0°C0000 0000 00 00 0000

+0.25°C0000 0000 01 00 0000

+1°C0000 0001 00 00 0000

+4°C0000 0100 00 00 0000

+127.75°C0111 1111 11 00 0000

One−Shot Register

The one−shot register is used to initiate a conversion and

comparison cycle when the ADT7461A is in standby mode,

after which the device returns to standby. Writing to the

one−shot register address (0x0F) causes the ADT7461A to

perform a conversion and comparison on both the internal

and the external temperature channels. This is not a data

that causes the one−shot conversion. The data written to this

address is irrelevant and is not stored.

Consecutive ALERT Register

The value written to this register determines how many

out−of−limit measurements must occur before an ALERT is

generated. The default value is that one out−of−limit

measurement generates an ALERT. The maximum value

that can be chosen is 4. The purpose of this register is to

allow the user to perform some filtering of the output. This

is particularly useful at the fastest three conversion rates,

where no averaging takes place. This register is at Address

0x22.

Table 8. Consecutive ALERT Register Codes

Measurements Required

yxxx 000x 1

yxxx 001x 2

yxxx 011x 3

yxxx 111x 4

NOTE: x = don’t care bits, and y = SMBus timeout bit.

Default = 0. See SMBus section for more information.

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Table 9. List of Registers

Read Address (Hex) Write Address (Hex) Name Power−On Default

Not Applicable Not Applicable Address Pointer Undefined

00 Not Applicable Local Temperature Value 0000 0000 (0x00)

01 Not Applicable External Temperature Value High Byte 0000 0000 (0x00)

02 Not Applicable Status Undefined

03 09 Configuration 0000 0000 (0x00)

04 0A Conversion Rate 0000 1000 (0x08)

05 0B Local Temperature High Limit 0101 0101 (0x55) (85°C)

06 0C Local Temperature Low Limit 0000 0000 (0x00) (0°C)

07 0D External Temperature High Limit High Byte 0101 0101 (0x55) (85°C)

08 0E External Temperature Low Limit High Byte 0000 0000 (0x00) (0°C)

Not Applicable 0F (Note 1) One−Shot

10 Not Applicable External Temperature Value Low Byte 0000 0000

11 11 External Temperature Offset High Byte 0000 0000

12 12 External Temperature Offset Low Byte 0000 0000

13 13 External Temperature High Limit Low Byte 0000 0000

14 14 External Temperature Low Limit Low Byte 0000 0000

19 19 External THERM Limit 0101 0101 (0x55) (85°C)

20 20 Local THERM Limit 0101 0101 (0x55) (85°C)

21 21 THERM Hysteresis 0000 1010 (0x0A) (10°C)

22 22 Consecutive ALERT 0000 0001 (0x01)

FE Not Applicable Manufacturer ID 0100 0001 (0x41)

FF Not Applicable Die Revision Code 0101 0111 (0x57)

1. Writing to Address 0x0F causes the ADT7461A to perform a single measurement. It is not a data register, and it does not matter what data

is written to it.

Serial Bus Interface

Control of the ADT7461A is carried out via the serial bus.

The ADT7461A is connected to this bus as a slave device,

under the control of a master device.

The ADT7461A has an SMBus timeout feature. When

this is enabled, the SMBus times out after typically 25 ms of

no activity. However, this feature is not enabled by default.

Bit 7 of the consecutive alert register (Address = 0x22)

should be set to enable it.

Addressing the Device

In general, every SMBus device has a 7−bit device

address, except for some devices that have extended 10−bit

addresses. When the master device sends a device address

over the bus, the slave device with that address responds.

The ADT7461Ais available with one device address, 0x4C

(1001 100b). An ADT7461A−2 is also available.

The ADT7461A−2 has an SMBus address of 0x4D (1001

101b). This is to allow two ADT7461A devices on the same

bus, or if the default address conflicts with an existing device

on the SMBus. The serial bus protocol operates as follows:

1. The master initiates a data transfer by establishing

a start condition, defined as a high−to−low

transition on SDATA, the serial data line, while

SCLK, the serial clock line, remains high. This

indicates that an address/data stream follows. All

slave peripherals connected to the serial bus

respond to the start condition and shift in the next

eight bits, consisting of a 7−bit address (MSB first)

plus an R/W bit, which determines the direction of

the data transfer, that is, whether data is written to,

or read from, the slave device. The peripheral

whose address corresponds to the transmitted

address responds by pulling the data line low

during the low period before the ninth clock pulse,

known as the acknowledge bit. All other devices

on the bus remain idle while the selected device

waits for data to be read from or written to it. If the

R/W bit is a 0, the master writes to the slave

device. If the R/W bit is a 1, the master reads from

the slave device.

2. Data is sent over the serial bus in a sequence of

nine clock pulses, eight bits of data followed by an

acknowledge bit from the slave device. Transitions

on the data line must occur during the low period

of the clock signal and remain stable during the

high period, since a low−to−high transition when

the clock is high can be interpreted as a stop

signal. The number of data bytes that can be

address is scm over xhc bus followed by R/W

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transmitted over the serial bus in a single read or

write operation is limited only by what the master

and slave devices can handle.

3. When all data bytes have been read or written,

stop conditions are established. In write mode, the

master pulls the data line high during the tenth

clock pulse to assert a stop condition. In read

mode, the master device overrides the

acknowledge bit by pulling the data line high

during the low period before the ninth clock pulse.

This is known as no acknowledge. The master

takes the data line low during the low period

before the tenth clock pulse, then high during the

tenth clock pulse to assert a stop condition.

Any number of bytes of data are transferable over

the serial bus in one operation, but it is not

possible to mix read and write in one operation

because the type of operation is determined at the

beginning and cannot subsequently be changed

without starting a new operation. For the

ADT7461A, write operations contain either one or

two bytes, while read operations contain one byte.

To write data to one of the device data registers, or to read

data from it, the address pointer register must be set so that

the correct data register is addressed. The first byte of a write

operation always contains a valid address that is stored in the

address pointer register. If data is to be written to the device,

the write operation contains a second data byte that is written

to the register selected by the address pointer register.

This procedure is illustrated in Figure 15. The device

address is sent over the bus followed by R/W set to 0. This

is followed by two data bytes. The first data byte is the

address of the internal data register to be written to, which

is stored in the address pointer register. The second data byte

is the data to be written to the internal data register.

Figure 15. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register

Figure 16. Writing to the Address Pointer Register Only

Figure 17. Reading from a Previously Selected Register

D7 D6 D5 D4 D3 D2 D1 D0

FRAME 3

DATA BYTE

ACK. BY

ADT7461A

STOP BY

MASTER

SDATA (CONTINUED)

SCLK (CONTINUED)

FRAME 1

SERIAL BUS ADDRESS BYTE

ACK. BY

ADT7461A

SDATA

SCLK

START BY

MASTER

FRAME 2

ADDRESS POINTER REGISTER BYTE

D7 D6 D5 D4 D3 D2 D1 D0A6 A5 A4 A3 A2 A1 A0 R/W

ACK. BY

ADT7461A

1 19 9

FRAME 1

SERIAL BUS ADDRESS BYTE

ACK. BY

ADT7461A

SDATA

SCLK

START BY

MASTER

FRAME 2

ADDRESS POINTER REGISTER BYTE

D7 D6 D5 D4 D3 D2 D1 D0A6 A5 A4 A3 A2 A1 A0 R/W

ACK. BY

ADT7461A

1 19 9

STOP BY

MASTER

FRAME 1

SERIAL BUS ADDRESS BYTE

ACK. BY

ADT7461A

SDATA

SCLK

START BY

MASTER FRAME 2

ADDRESS POINTER REGISTER BYTE

D7 D6 D5 D4 D3 D2 D1 D0A6 A5 A4 A3 A2 A1 A0 R/W

ACK. BY

ADT7461A

1 19 9

STOP BY

MASTER

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When reading data from a register there are two

possibilities.

•If the address pointer register value of the ADT7461A

is unknown or not the desired value, it is first necessary

to set it to the correct value before data can be read

from the desired data register. This is done by writing

to the ADT7461A as before, but only the data byte

containing the register read address is sent, because

data is not to be written to the register see Figure 16.

A read operation is then performed consisting of the

serial bus address, R/W bit set to 1, followed by the

data byte read from the data register see Figure 17.

•If the address pointer register is known to be at the

desired address, data can be read from the

corresponding data register without first writing to the

address pointer register and the bus transaction shown

in Figure 16 can be omitted.

Notes:

•It is possible to read a data byte from a data register

without first writing to the address pointer register.

However, if the address pointer register is already at the

correct value, it is not possible to write data to a register

without writing to the address pointer register because

the first data byte of a write is always written to the

address pointer register.

•Some of the registers have different addresses for read

and write operations. The write address of a register

must be written to the address pointer if data is to be

written to that register, but it may not be possible to

read data from that address. The read address of a

data can be read from that register.

ALERT Output

This is applicable when Pin 6 is configured as an ALERT

output. The ALERT output goes low whenever an

out−of−limit measurement is detected, or if the remote

temperature sensor is open circuit. It is an open−drain output

and requires a pullup resistor. Several ALERT outputs can

be wire−OR’ed together, so that the common line goes low

if one or more of the ALERT outputs goes low.

The ALERT output can be used as an interrupt signal to a

processor, or as an SMBALERT. Slave devices on the SMBus

cannot normally signal to the bus master that they want to

talk, but the SMBALERT function allows them to do so.

One or more ALERT outputs can be connected to a

common SMBALERT line that is connected to the master.

When the SMBALERT line is pulled low by one of the

devices, the following procedure occurs (see Figure 18):

Figure 18. Use of SMBALERT

ALERT RESPONSE

ADDRESS

MASTER SENDS

ARA AND READ

COMMAND DEVICE SENDS

ITS ADDRESS

RDSTART ACK DEVICE

ADDRESS

ACK STOP

MASTER

RECEIVES

SMBALERT

1. SMBALERT is pulled low.

2. Master initiates a read operation and sends the

alert response address (ARA = 0001 100). This is

a general call address that must not be used as a

specific device address.

3. The device whose ALERT output is low responds

to the alert response address and the master reads

its device address. As the device address is seven

bits, an LSB of 1 is added. The address of the

device is now known and it can be interrogated in

the usual way.

4. If more than one device’s ALERT output is low,

the one with the lowest device address takes

priority, in accordance with normal SMBus

arbitration.

Once the ADT7461A has responded to the alert response

address, it resets its ALERT output, provided that the error

condition that caused the ALERT no longer exists. If the

SMBALERT line remains low, the master sends the ARA

again, and so on until all devices whose ALERT outputs

were low have responded.

Low Power Standby Mode

The ADT7461A can be put into low power standby mode

by setting Bit 6 of the configuration register. When Bit 6 is

low, the ADT7461A operates normally. When Bit 6 is high,

the ADC is inhibited, and any conversion in progress is

terminated without writing the result to the corresponding

value register. However, the SMBus is still enabled. Power

consumption in the standby mode is reduced to 5 mA if there

is no SMBus activity, or 30 mA if there are clock and data

signals on the bus.

When the device is in standby mode, it is possible to

initiate a one−shot conversion of both channels by writing to

the one−shot register (Address 0x0F), after which the device

returns to standby. It does not matter what is written to the

one−shot register, all data written to it is ignored. It is also

possible to write new values to the limit register while in

standby mode. If the values stored in the temperature value

registers are outside the new limits, an ALERT is generated,

even though the ADT7461A is still in standby.

AiLERT ﬂag on ses AiLERT ALERT THERM ALERT THERM AiLERT AiLERT to assert. iALERT THERM THERM THERM THERM THERM 35 ° 11 caseiTHERM e hysteresis lump on (he THERM THERM 's system can be setup so that when THERM THERM re the temperature hnvers around the THERM Table 10. THERM THERE THERM AL RT ALERT SMBALERT risenr The user can use the THERM \ 7 Is / HIGH ram: LIMI'I eKEER‘r THEFM up temperature limit, the ALERT THERM limit, the THERM The THERM temperature falls to THERM The ALERT ALERT output or as an additional THERM THERMZ limits. It is reset in the same manner as THERM THERMZ THERM THERMZ In (his example, the THERMZ THERM THERMZ THERM THERM

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Sensor Fault Detection

At its D+ input, the ADT7461A contains internal sensor

fault detection circuitry. This circuit can detect situations

where an external remote diode is either not connected or

incorrectly connected to the ADT7461A. A simple voltage

comparator trips if the voltage at D+ exceeds VDD − 1 V

(typical), signifying an open circuit between D+ and D−.

The output of this comparator is checked when a conversion

is initiated. Bit 2 of the status register (open flag) is set if a

fault is detected. If the ALERT pin is enabled, setting this

flag causes ALERT to assert low.

If the user does not wish to use an external sensor with the

ADT7461A, tie the D+ and D− inputs together to prevent

continuous setting of the open flag.

The ADT7461A Interrupt System

The ADT7461A has two interrupt outputs, ALERT and

THERM. Both have different functions and behavior.

ALERT is maskable and responds to violations of software

programmed temperature limits or an open−circuit fault on

the external diode. THERM is intended as a fail−safe

interrupt output that cannot be masked.

If the external or local temperature exceeds the

programmed high temperature limits, or equals or exceeds

the low temperature limits, the ALERT output is asserted

low. An open−circuit fault on the external diode also causes

ALERT to assert. ALERT is reset when serviced by a master

reading its device address, provided the error condition has

gone away and the status register has been reset.

The THERM output asserts low if the external or local

temperature exceeds the programmed THERM limits.

THERM temperature limits should normally be equal to or

greater than the high temperature limits. THERM is reset

automatically when the temperature falls back within the

THERM limit. The external and local limits are set by

default to 85°C. A hysteresis value can be programmed; in

which case, THERM resets when the temperature falls to the

limit value minus the hysteresis value. This applies to both

local and remote measurement channels. The power−on

hysteresis default value is 10°C, but this can be

reprogrammed to any value after powerup.

The hysteresis loop on the THERM outputs is useful when

THERM is used, for example, as an on/off controller for a

fan. The user’s system can be set up so that when THERM

asserts, a fan is switched on to cool the system. When

THERM goes high again, the fan can be switched off.

Programming a hysteresis value protects from fan jitter,

where the temperature hovers around the THERM limit, and

the fan is constantly switched.

Table 10. THERM Hysteresis

THERM Hysteresis Binary Representation

0°C0 000 0000

1°C0 000 0001

10°C0 000 1010

Figure 19 shows how the THERM and ALERT outputs

operate. The ALERT output can be used as a SMBALERT

to signal to the host via the SMBus that the temperature has

risen. The user can use the THERM output to turn on a fan

to cool the system, if the temperature continues to increase.

This method ensures that there is a fail−safe mechanism to

cool the system, without the need for host intervention.

Figure 19. Operation of the ALERT and THERM

Interru

HIGH TEMP LIMIT

RESET BY MASTER

ALERT

THERM

1005C

TEMPERATURE

905C

805C

705C

605C

505C

405C

THERM LIMIT

THERM LIMIT−HYSTERESI

•If the measured temperature exceeds the high

temperature limit, the ALERT output asserts low.

•If the temperature continues to increase and exceeds the

THERM limit, the THERM output asserts low. This can

be used to throttle the CPU clock or switch on a fan.

•The THERM output deasserts (goes high) when the

temperature falls to THERM limit minus hysteresis. In ,

the default hysteresis value of 10°C is shown.

•The ALERT output deasserts only when the

temperature has fallen below the high temperature

limit, and the master has read the device address and

cleared the status register.

•Pin 6 on the ADT7461A can be configured as either an

ALERT output or as an additional THERM output.

•THERM2 asserts low when the temperature exceeds the

programmed local and/or remote high temperature

limits. It is reset in the same manner as THERM and is

not maskable.

•The programmed hysteresis value also applies to

THERM2.

Figure 20 shows how THERM and THERM2 operate

together to implement two methods of cooling the system.

In this example, the THERM2 limits are set lower than the

THERM limits. The THERM2 output is used to turn on a

fan. If the temperature continues to rise and exceeds the

THERM limits, the THERM output provides additional

cooling by throttling the CPU.

Figure 20. Operation of (he THERM and THERMZ When ‘hc THERMZ limil is exceeded, lhc THERMZ THERM limh, ‘hc THERM The THERM temperature falls m THERM bclnw lhc THERMZ limh, ‘hc THERMZ Again, no hyslcrmis value is shown for THERMZ cause THERM and THERMZ 1mm

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Figure 20. Operation of the THERM and THERM2

Interrupts

THERM2

THERM LIMIT

THERM2 LIMIT

TEMPERATURE

THERM

905C

805C

705C

605C

505C

405C

305C

•When the THERM2 limit is exceeded, the THERM2

signal asserts low.

•If the temperature continues to increase and exceeds the

THERM limit, the THERM output asserts low.

•The THERM output deasserts (goes high) when the

temperature falls to THERM limit minus hysteresis. In

Figure 20, there is no hysteresis value shown.

•As the system cools further, and the temperature falls

below the THERM2 limit, the THERM2 signal resets.

Again, no hysteresis value is shown for THERM2.

Both the external and internal temperature measurements

cause THERM and THERM2 to operate as described.

Application Information

Noise Filtering

For temperature sensors operating in noisy environments,

the industry standard practice was to place a capacitor across

the D+ and D− pins to help combat the effects of noise.

However, large capacitances affect the accuracy of the

temperature measurement, leading to a recommended

maximum capacitor value of 1,000 pF. Although this

capacitor reduces the noise, it does not eliminate it, making

it difficult to use the sensor in a very noisy environment.

The ADT7461A has a major advantage over other devices

when it comes to eliminating the effects of noise on the

external sensor. The series resistance cancellation feature

allows a filter to be constructed between the external

temperature sensor and the part. The effect of any filter

resistance seen in series with the remote sensor is

automatically cancelled from the temperature result.

The construction of a filter allows the ADT7461A and the

remote temperature sensor to operate in noisy environments.

Figure 21 shows a low−pass R−C−R filter, where R = 100 W

and C = 1 nF. This filtering reduces both common−mode and

differential noise.

Figure 21. Filter Between Remote Sensor and

ADT7461A Factors Affecting Diode Accuracy

D+

1nF

100W

REMOTE

TEMPERATURE

SENSOR

D–

100W

Remote Sensing Diode

The ADT7461A is designed to work with substrate

transistors built into processors or with discrete transistors.

Substrate transistors are generally PNP types with the

collector connected to the substrate. Discrete types are either

PNP or NPN transistors connected as diodes (base−shorted

to collector). If an NPN transistor is used, the collector and

base are connected to D+ and the emitter to D−. If a PNP

transistor is used, the collector and base are connected to D−

and the emitter to D+.

To reduce the error due to variations in both substrate and

discrete transistors, consider several factors:

•The ideality factor, nF, of the transistor is a measure of

the deviation of the thermal diode from ideal behavior.

The ADT7461A is trimmed for an nF value of 1.008.

The following equation may be used to calculate the

error introduced at a temperature, T (°C), when using a

transistor whose nF does not equal 1.008. Consult the

processor data sheet for the nF values.

DT = (nF − 1.008)/1.008 x (273.15 Kelvin + T)

To factor this in, the user writes the DT value to the offset

from, the temperature measurement.

•Some CPU manufacturers specify the high and low

current levels of the substrate transistors. The high

current level of the ADT7461A, IHIGH, is 220 mA and

the low level current, ILOW, is 13.5 mA. If the

ADT7461A current levels do not match the current

levels specified by the CPU manufacturer, it may

become necessary to remove an offset. The CPU data

sheet should advise whether this offset needs to be

removed and how to calculate it. This offset is

programmed to the offset register. It is important to

note that if more than one offset must be considered,

the algebraic sum of these offsets must be programmed

to the offset register.

If a discrete transistor is used with the ADT7461A, the

best accuracy is obtained by choosing devices according to

the following criteria:

•Base−emitter voltage greater than 0.25 V at 6 mA, at the

highest operating temperature

•Base−emitter voltage less than 0.95 V at 100 mA, at the

lowest operating temperature

•Base resistance less than 100 W

•Small variation in hFE (50 to 150) that indicates tight

control of VBE characteristics

Transistors, such as the 2N3904, 2N3906, or equivalents

in SOT−23 packages are suitable devices to use.

Thermal Inertia and Self−Heating

Accuracy depends on the temperature of the remote

sensing diode and/or the internal temperature sensor being

at the same temperature as that being measured. Many

factors can affect this. Ideally, place the sensor in good

thermal contact with the part of the system being measured.

THERM ALERT

ADT7461A

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If it is not, the thermal inertia caused by the sensor’s mass

causes a lag in the response of the sensor to a temperature

change. In the case of the remote sensor, this should not be

a problem since it is either a substrate transistor in the

processor or a small package device, such as the SOT−23,

placed in close proximity to it.

The on−chip sensor, however, is often remote from the

processor and only monitors the general ambient

temperature around the package. How accurately the

temperature of the board and/or the forced airflow reflects

the temperature to be measured dictates the accuracy of the

measurement. Self−heating due to the power dissipated in

the ADT7461A or the remote sensor causes the chip

temperature of the device or remote sensor to rise above

ambient. However, the current forced through the remote

sensor is so small that self−heating is negligible. In the case

of the ADT7461A, the worst−case condition occurs when

the device is converting at 64 conversions per second while

sinking the maximum current of 1 mA at the ALERT and

THERM output. In this case, the total power dissipation in

the device is about 4.5 mW. The thermal resistance, qJA, of

the 8−lead MSOP is approximately 142°C/W.

Layout Considerations

Digital boards can be electrically noisy environments, and

the ADT7461A is measuring very small voltages from the

remote sensor, so care must be taken to minimize noise

induced at the sensor inputs. Take the following precautions:

•Place the ADT7461A as close as possible to the remote

sensing diode. Provided that the worst noise sources,

that is, clock generators, data/address buses, and CRTs

are avoided, this distance can be 4 inches to 8 inches.

•Route the D+ and D– tracks close together, in parallel,

with grounded guard tracks on each side. To minimize

inductance and reduce noise pickup, a 5 mil track width

and spacing is recommended. Provide a ground plane

under the tracks, if possible.

Figure 22. Typical Arrangement of Signal Tracks

GND

D+

D–

GND

5 MIL

•Try to minimize the number of copper/solder joints that

can cause thermocouple effects. Where copper/solder

joints are used, make sure that they are in both the D+

and D− path and at the same temperature.

•Thermocouple effects should not be a major problem as

1°C corresponds to about 200 mV, and thermocouple

voltages are about 3 mV/°C of temperature difference.

Unless there are two thermocouples with a big

temperature differential between them, thermocouple

voltages should be much less than 200 mV.

•Place a 0.1 mF bypass capacitor close to the VDD pin. In

extremely noisy environments, place an input filter

capacitor across D+ and D− close to the ADT7461A.

This capacitance can effect the temperature

measurement, so ensure that any capacitance seen at D+

and D− is, at maximum, 1,000 pF. This maximum value

includes the filter capacitance, plus any cable or stray

capacitance between the pins and the sensor diode.

•If the distance to the remote sensor is more than 8

inches, the use of twisted pair cable is recommended. A

total of 6 feet to 12 feet is needed.

For really long distances (up to 100 feet), use a shielded

twisted pair, such as the Belden No. 8451 microphone

cable. Connect the twisted pair to D+ and D− and the

shield to GND close to the ADT7461A. Leave the

remote end of the shield unconnected to avoid ground

loops.

Because the measurement technique uses switched

current sources, excessive cable or filter capacitance can

affect the measurement. When using long cables, the filter

capacitance can be reduced or removed.

Application Circuit

Figure 23 shows a typical application circuit for the

ADT7461A, using a discrete sensor transistor connected via

a shielded, twisted pair cable. The pullups on SCLK,

SDATA, and ALERT are required only if they are not

provided elsewhere in the system.

The SCLK pin and the SDATA pin of the ADT7461A can

be interfaced directly to the SMBus of an I/O controller, such

as the IntelR 820 chipset..

61A

ADT7461A

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Figure 23. Typical Application Circuit

5V OR 12V

SMBUS

CONTROLLER

FAN CONTROL

CIRCUIT

2N3906

CPU THERMAL

DIODE

D+

Dć

VDD

SCLK

SDATA

ALERT/

THERM2

THERM

GND

ADT7461A 0.1mF

VDD

TYP 10k

FAN ENABLE

3V TO 3.6V

TYP 10k

SHIELD

ORDERING INFORMATION

Device Order Number*Package Description Branding SMBus Address Shipping†

ADT7461AARMZ

8−Lead MSOP

T1K 4C

50 Tube

ADT7461AARMZ−R3000 Tape & Reel

ADT7461AARMZ−RL7 1000 Tape & Reel

ADT7461AARMZ−002

T1L 4D

50 Tube

ADT7461AARMZ−2RL 3000 Tape & Reel

ADT7461AARMZ2RL7 1000 Tape & Reel

†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging

Specifications Brochure, BRD8011/D.

*The “Z’’ suffix indicates Pb−Free package available.

a—ﬁfma TE Ale i ﬂ‘ki new Una Una oszSc um ums mm 0qu ans um m9: may 0N Semxcanduclnvand J5 manymumm scum:

ADT7461A

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PACKAGE DIMENSIONS

0.08 (0.003) A S

NOTES:

1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982.

2. CONTROLLING DIMENSION: MILLIMETER.

3. DIMENSION A DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE

BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED

0.15 (0.006) PER SIDE.

4. DIMENSION B DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSION.

INTERLEAD FLASH OR PROTRUSION SHALL NOT EXCEED 0.25 (0.010) PER SIDE.

5. 846A-01 OBSOLETE, NEW STANDARD 846A-02.

PIN 1 ID

8 PL

0.038 (0.0015)

−T−

SEATING

PLANE

A1 cL

DIM

MIN NOM MAX MIN

MILLIMETERS

−− −− 1.10 −−

INCHES

A1 0.05 0.08 0.15 0.002

b0.25 0.33 0.40 0.010

c0.13 0.18 0.23 0.005

D2.90 3.00 3.10 0.114

E2.90 3.00 3.10 0.114

e0.65 BSC

L0.40 0.55 0.70 0.016

−− 0.043

0.003 0.006

0.013 0.016

0.007 0.009

0.118 0.122

0.026 BSC

0.021 0.028

NOM MAX

4.75 4.90 5.05 0.187 0.193 0.199

MSOP8

CASE 846AB−01

ISSUE O

*For additional information on our Pb−Free strategy and soldering

details, please download the ON Semiconductor Soldering and

Mounting Techniques Reference Manual, SOLDERRM/D.

SOLDERING FOOTPRINT*

8X 8X

6X ǒmm

inchesǓ

SCALE 8:1

1.04

0.041

0.38

0.015

5.28

0.208

4.24

0.167

3.20

0.126

0.65

0.0256

ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice

to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability

arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.

“Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All

operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights

nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications

intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should

Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates,

and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death

associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal

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PUBLICATION ORDERING INFORMATION

N. American Technical Support: 800−282−9855 Toll Free

USA/Canada

Europe, Middle East and Africa Technical Support:

Phone: 421 33 790 2910

Japan Customer Focus Center

Phone: 81−3−5773−3850

ADT7461A/D

Protected by U.S. Patents 5,195,827; 5,867,012; 5,982,221; 6,097,239; 6,133,753; 6,169,442; 7,010,440 ; other patents pending.

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