Diagnosing LEDs by Monitoring Switching Mode Duty Cycle

Engineers typically monitor the forward voltage VF of HB LEDs (high brightness light emitting diodes) to determine if an LED is good or bad. Large changes in forward voltage may indicate that one or a string of LEDs has degraded in quality or is completely damaged. For multiple LEDs in series, the sum of their forward voltages can reach more than 40V, and differential measurement is required if the voltage is not referenced to ground. In addition to the challenges of large voltages and differential measurements, HB LEDs often use PWM (pulse width modulation) for dimming. If this is the case, the forward voltage cannot be measured during low PWM duty cycles because the LED is not emitting light and there is no forward voltage. For a hysteretic buck LED driver driving three LEDs in series, the voltage between the positive and negative poles of the connected string must be measured when the Dim pin is high.

Figure 1 For a hysteretic buck LED driver driving three LEDs in series, the positive and negative voltages must be measured when the Dim pin is high.

To avoid the requirement for differential high-voltage measurements, the duty cycle can be measured directly at the DRV pin. For this LED driver, a first-order estimate of the LED string forward voltage is VF = D × VIN, where D is the internal duty cycle generated by the switch-mode portion of the IC; do not confuse this duty cycle with the Dim pin. Reference the drive signal to ground and limit it to the 5V supply voltage, VCC. This allows the use of a low-voltage ADC or comparator, which can be powered by the LED driver's VCC output (10 mA maximum).

Figure 2 Adding this comparator circuit to the circuit in Figure 1 can detect a short-circuited LED.

Figure 2 shows how a comparator can help detect a shorted LED. Filter R1C1 converts the driver's AC PWM signal to a DC voltage VD that is proportional to D × VCC. VD should be sampled when it is greater than 90% of its possible stable value; this sampling requires at least 2.3R1C1 cycles. Since the comparator latches the output when LE (latch enable) is low, LE should be asserted no earlier than 2.3R1C1 after the Dim pin goes high. R2, C2, and D2 ensure that LE is deactivated immediately when Dim goes low.
The value of R2C2 is greater than that of R1C1, so the comparator is enabled when the input signal reaches at least 90% of its steady-state value. D2 discharges C2 immediately after the Dim pin goes low, latching the output as soon as the LED turns off.


Figure 3. For a system with 12V input voltage and three series LEDs, the LED forward voltage is approximately 3V (a), and the filtered driver signal (green) is stable at approximately D×VCC=(9V/12V)5V=3.75V. The comparator latches when the filtered Dim signal (yellow) falls below 2.5V, and the comparator begins to resolve the filtered driver signal about 100μs later. Obviously, when the comparator is valid, VD is above the threshold reference voltage (red). After an LED shorts (b), VD stabilizes at approximately (6V/12V)5V=2.5V, no longer above the threshold voltage.

Since the reference voltage is lower than D×VIN, the comparator output is normally low. If an LED shorts due to a fault, its forward voltage drops, causing the duty cycle of the driver to decrease. VD then drops below the reference voltage, causing the comparator output to be high, indicating a shorted LED. Since the output is latched when the Dim pin is low, the fault signal is maintained even if the LED is powered off. Figure 3 shows the filtered Dim pin and driver signals for normal operation and when there is a shorted LED.

For a 12V input voltage and three LEDs in series, the forward voltage of the LEDs is about 3V (Figure 3a), and the filtered driver signal (green) settles at about D×VCC=(9V/12V)5V=3.75V. The comparator latches when the filtered Dim signal (yellow) falls below 2.5V, so the comparator starts resolving the filtered driver signal after about 100μs. Obviously, when the comparator activates, VD is above the threshold reference voltage (red). When one LED is shorted (Figure 3b), VD settles to about (6V/12V)5V=2.5V and is no longer above the threshold voltage. This condition causes the comparator output to be high, indicating that one LED is shorted.

The selection of the filter constant depends on several parameters. The cutoff frequency should be low enough to properly filter the driver signal, which is small enough so that the filtered signal can settle to a steady-state value close to that achievable within the shortest dimming pulse. This circuit can be easily adapted to detect open LEDs. When an LED fails and stops conducting current, the driver duty cycle reaches 100% when the Dim pin is high. Then, if the comparator input connections are swapped so that the reference voltage is slightly lower than VCC, the comparator output will be high when the LED is open.