shows how to detect a short-circuited LED with the aid of a comparator (
MAX9141). Filter R
1C
1 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 2.3R
1C
1. Because the comparator's Latch Enable (LE) latches the output when LE is low, LE should assert not earlier than 2.3R
1C
1 after DIM goes high. R
2C
2 in combination with D
2 ensures that LE de-asserts immediately after DIM goes low. The R
2C
2 value is higher than R
1C
1, so the comparator enables when the input signal reaches at least 90% of its steady-state value. D
2 discharges C
2 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.
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