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shows how to detect a short-circuited LED with the aid of a comparator (MAX9141). Filter R1C1 converts the AC PWM signal at DRV to a DC voltage (VD) proporTIonal to D × VCC. VD should be sampled when its value is greater than (perhaps) 90% of its steady-state value, which requires a period of at least 2.3R1C1. Because the comparator's Latch Enable (LE) latches the output when LE is low, LE should assert not earlier than 2.3R1C1 after DIM goes high. R2C2 in combinaTIon with D2 ensures that LE de-asserts immediately after DIM goes low. The R2C2 value is higher than R1C1, so the comparator enables when the input signal reaches at least 90% of its steady-state value. D2 discharges C2 immediately after DIM goes low, which latches the output as soon as the LEDs go off.


Figure 2. Adding this comparator circuit to the Figure 1 circuit provides detecTIon of shorted LEDs.