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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 (V_D) proportional to D × V_CC. V_D 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.