Time

VBOOST

Driver

headroom

VBOOST

LP8553

SNVS701B – SEPTEMBER 2011 – REVISED JANUARY 2014

www.ti.com

Figure 20. Boost Adaptive Control Principle with PSPWM

Fault Detection

LP8553 has fault detection for LED fault, low-battery voltage, over-current and thermal shutdown. The open drain

output pin (FAULT) can be used to indicate occurred fault. The cause for the fault can be read from status

register. Reading the fault register will also reset the fault. Setting the EN pin low will also reset the faults, even if

an external 5V line is used to power VLDO pin.

LED Fault Detection

With LED fault detection, the voltages across the LED drivers are constantly monitored. Shorted or open LED

string is detected.

If LED fault is detected:

•

The corresponding LED string is taken out of boost adaptive control loop;

•

Fault bits are set in the fault register to identify whether the fault has been open/short and how many strings

are faulty; and

•

Fault open-drain pin is pulled down.

LED fault sensitivity can be adjusted with <LED_FAULT_THR> EEPROM bit which sets the allowable variation

between LED output voltage to 3.3V or 5.3V. Depending on application and how much variation there can be in

normal operation between LED string forward voltages this setting can be adjusted.

Fault is cleared by setting EN pin low or by reading the fault register.

By default the LED fault detection is active only in automatic PWM & current dimming mode. If LED fault

detection is needed in PWM dimming mode, please contact TI representative for guidance.

Under-Voltage Detection

LP8553 has detection for too-low VIN voltage. Threshold level for the voltage is set with EEPROM register bits

as seen in the following table:

UVLO[1:0]

Threshold (V)

00

OFF

01

2.7V

10

5.4V

11

8.1V

When under voltage is detected the LED outputs and boost will shutdown, FAULT pin is pulled down and

corresponding fault bit is set in fault register. LEDs and boost will start again when the voltage has increased

above the threshold level. Hysteresis is implemented to threshold level to avoid continuous triggering of fault

when threshold is reached.

Fault is cleared by setting EN pin low or by reading the fault register.

20

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