10.2.3 PWM Dimming by Modulated Supply

The TPS92638-Q1 device supports PWM dimming from the supply as depicted below. A high-side switch in the body control module (BCM) usually implements supply dimming. Due to the nature of the high-side switch, TPS92638-Q1 supply voltage is not strongly pulled down to ground, but depends on the decoupling capacitor and total current consumption. The TPS92638-Q1 device keeps the output current constant as long as supply voltage is adequate to overcome the LED forward voltage and dropout voltage.

When supply voltage drops too low to drive LEDs, the device shuts down the output channels on open-load detection. Therefore, TI recommends ensuring channel shutdown using the PWM or EN inputs. Thus a resistor string of R1 and R2 is recommended to ensure the lowest divided voltage is lower than PWM threshold.

Figure 42. Schematic for PWM Dimming by Modulated Supply