The IS32LT3175 is a linear, single channel constant current source incorporating resistor programmable features. A single resistor, RISET programs the current source to a value between 10mA to 150mA. Two other resistors set the theatrical dimming speed, RTSET_UP sets the ON dimming time while RTSET_DN sets the OFF dimming time. There are two methods to control the LED dimming; with a momentary contact switch or with a BCM PWM pulse stream input. Automobiles have a microcontroller in a Body Control Module which will send out a pulse-width-modulated (PWM) dimming signal when a door is opened or closed. This PWM signal will directly drive the LT3175 current source to gradually turn ON the LED when a door is opened and gradually turn it OFF when it is closed. Because the BCM PWM signal, sometimes referred to as the courtesy light signal, can be positive or negative polarity, Lumissil offers two versions; LT3175P for positive polarity (most common) and LT3175N for negative polarity.