Low threshold

Mid threshold (Low + 0.5V)

High threshold

This causes the

open fault!

LP8543

SNVS604D – AUGUST 2009 – REVISED MARCH 2013

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Led Fault Detection

There are two methods of detecting the LED fault. First method is based on measuring the voltage on LED driver

pins (analog fault detection) and another is based on adaptive boost voltage hopping between strings (digital

fault detection). The used fault detection mode is selected in EEPROM as well as the threshold levels.

<FAULT_SEL[1:0]> bits selects the used mode as follows:

Table 9. LED Fault Mode Selection

FAULT_SEL[1:0]

Fault mode

00

No fault detection

01

Analog fault detection based on LED driver voltage

10

Digital fault detection based on boost voltage hopping

11

Both analog and digital fault detection

Two fault detection methods are used to detect faults in different conditions. Analog detection works better with

high PWM values (in PSPWM mode) where many strings are active at a same time. It does not work when only

one string is active at a time, because it is based on comparing driver voltages on strings active simultaneously.

Digital fault detection is used to complement this case.

Digital fault detection works better with low PWM values, where not all strings are on at the same time. Digital

short detection works only with cases where one string is active at the same time.

Analog Fault Detection

When analog fault detection mode is selected, the voltages across the LED drivers are constantly monitored. The

same threshold levels (Low, Mid and High) are used for fault detection to adjust the boost voltage.

If one of the LED strings has an open fault (LED driver output pin has no contact to LED string), the output pin

voltage drops to 0V. When this happens the boost voltage will be adjusted higher to get enough headroom, but

at some point the voltage for all other strings will rise over the high threshold. In this case the LP8543 detects

open fault, and adjusts the boost voltage based on other LED strings needs, i.e., the faulty LED string voltage is

not used anymore for adjusting boost output voltage. If the LED driver output pin is shorted to GND the fault

detection works exactly the same. This situation with 6 LEDs active at the same time is illustrated in the following

diagram:

Figure 21. Open Fault

If one or more LEDs are shorted, this causes the voltage to rise in this LED driver output pin above the high

threshold. This causes short fault detection as seen in the following figure:

26

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