Time

VBOOST

Driver

headroom

VBOOST

LP8550

www.ti.com

SNVS657E – SEPTEMBER 2010 – REVISED SEPTEMBER 2014

VBOOST[4:0]

VOLTAGE (typical)

...

...

...

11101

29

39

11110

30

40

11111

31

40

8.3.3.4 Adaptive Boost Control

Adaptive boost control function adjusts the boost output voltage to the minimum sufficient voltage for proper LED

driver operation. The output with highest VF LED string is detected and boost output voltage adjusted

accordingly. Driver headroom can be adjusted with <DRIVER_HEADR[2:0]> EEPROM bits from approximately

300 mV to 1200 mV. Boost adaptive control voltage step size is 125 mV. Boost adaptive control operates

similarly with and without PSPWM.

Figure 18. Boost Adaptive Control Principle With PSPWM

8.3.4 Fault Detection

The LP8550 has fault detection for LED fault, low-battery voltage, overcurrent 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 also resets the fault. Setting the EN pin low also resets the faults, even if an

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

8.3.4.1 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 3.3 V or 5.3 V. 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 a TI representative for guidance.

Copyright © 2010–2014, Texas Instruments Incorporated

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Product Folder Links: LP8550