PSPWM cycle

Boost voltage

>Threshold

Shorted LEDs

in this string

PSPWM cycle

Boost voltage

>Threshold

38V

Open fault in

this string

Low threshold

Mid threshold (Low + 0.5V)

High threshold

This causes short fault!

LP8543

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SNVS604D – AUGUST 2009 – REVISED MARCH 2013

Figure 22. Short Fault

Digital Fault Detection

With digital fault detection the voltage hopping between LED strings is monitored in PSPWM mode. In normal

PWM mode or with high PWM values with PSPWM mode this does not apply.

If there’s open in one of the LED strings, the LED driver output pin will drop to 0V. When this happens the boost

will try to increase the voltage to get enough headroom for the driver. When the voltage for one string reaches

maximum voltage (38V) and the difference between consecutive LED strings is higher than set threshold level an

open LED fault is detected. If all voltages are close to 38V then the threshold condition is not met and no fault is

detected. If the LED output is shorted to GND it will be detected same way. Open fault detection is seen in the

following figure:

Figure 23. Digital Open Fault Detection

If there is one or more LEDs shorted in one string, the boost will drop the voltage for this string. When the

difference between consecutive LED strings is higher than set threshold level a short LED fault is detected. This

is described in the following figure:

Figure 24. Digital Short Fault Detection

Threshold level is programmed to EEPROM as shown in the following table. Threshold level adjustability is

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