Diagnose LEDs by monitoring the switch-mode du

A change in the forward voltage (V_F) for a string of series-connected, high-brightness LEDs (HB LEDs) can indicate deterioration or complete failure of one or more of the LEDs. Accordingly, this circuit assesses the LED's health bymonitoringV_F.
The forward voltage (V_F) of high-brightness LEDs (HB LEDs) is often monitored to assess the health of the LED. Big changes in V_Fcan indicate deterioration or even a complete failure of one or more LEDs connected in series.

For several LEDs in series, the sum of their V_Fvoltages can go to 40V or more, and if not referenced to ground, that V_Fsum requires a differential measurement. As a third challenge (in addition to high voltage and differential measurement), HBLEDs are often dimmed using pulse-width modulation (PWM). If so, you can't measure V_Fduring the low portion of the PWM dutycycle, when the LEDs are not illuminated and V_Fis not present. For a hysteretic buck LED driver (MAX16820) driving three LEDs in series (Figure 1), you must measure the anode and cathode voltages of the string when DIM is high.


Figure 1. Standard driver circuit for HB LEDs.

To avoid the need for a differential high-voltage measurement, you can take the indirect approach of measuring the dutycycleat the DRV pin. For this particular LED driver, a first-order estimate of forward voltage for the LED string is V_F= D × V_IN, where D represents an internal dutycycleproduced in the IC's switchmode section (not to be confused with the duty cycle at DIM). The DRV signal is referenced to ground and limited to V_CC(5V). That condition allows the use of low voltage ADCs or comparators, which in turn can be powered by the LED driver's V_CCoutput (10mA maximum).

Figure 2shows how to detect a short-circuited LED with the aid of a comparator (MAX9141). Filter R₁C₁converts the AC PWM signal at DRV to a DC voltage (V_D) proportional to D × V_CC. V_Dshould 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 2.3R₁C₁after DIM goes high. R₂C₂in combination with D₂ensures that LE de-asserts immediately after DIM goes low. The R₂C₂value is higher than R₁C₁, so the comparator enables when the input signal reaches at least 90% of its steady-state value. D₂discharges C₂immediately 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.

Because Ref is lower than D × V_IN, the comparator output is normally low. If an LED fails shorted, its forward voltage drops and causes the duty cycle at DRV to drop. V_Dthen drops below Ref, causing the comparator output to go high, indicating a shorted LED. Because the output latches when DIM goes low, the error signal remains asserted even when the LEDs are off.Figure 3shows the filtered DIM and DRV signals for normal operation vs. a shorted-LED condition.