LP8550TLESLASHNOPB.A datasheet(11/49 Pages) TI2

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LP8550

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SNVS657E – SEPTEMBER 2010 – REVISED SEPTEMBER 2014

8 Detailed Description

8.1 Overview

LP8550 is a high voltage LED driver for medium-sized LCD backlight applications. It includes high voltage boost

converter. Boost voltage automatically sets to the correct level needed to drive the LED strings. This is done by

monitoring LED output voltage drop in real time.

Six LED outputs are driven either with constant current sinks with PWM control or by controlling both PWM and

current. Constant current value is set with EEPROM bits and with RISET resistor. Brightness (PWM) is controlled

either with I2C register or with PWM input. PWM frequencies are set with EEPROM bits and with RFSET resistor.

Special Phase-Shift PWM mode can be used to reduce boost output current peak, thus reducing output ripple,

capacitor size and audible noise.

With LP8550 it is possible to synchronize the PWM output frequency to VSYNC signal received from video

processor. Internal PLL ensures that the PWM output clock is always synchronized to the VSYNC signal.

Special dithering mode makes it possible to increase output resolution during fading between two brightness

values and by this making the transition look very smooth with virtually no stepping. Transition slope time can be

adjusted with EEPROM bits.

Safety features include LED fault detection with open and short detection. LED fault detection prevents system

overheating in case of open in some of the LED strings. Chip internal temperature is constantly monitored and

based on this LP8550 can reduce the brightness of the backlight to reduce thermal loading once certain trip point

is reached. Threshold is programmable in EEPROM. If chip internal temperature reaches too high, the boost

converter and LED outputs are completely turned off until the internal temperature has reached acceptable level.

Boost converter is protected against too high load current and over-voltage. LP8550 notifies the system about

the fault through I2C register and with FAULT pin.

EEPROM programmable functions include:

•

PWM frequencies

•

Phase shift PWM mode

•

LED constant current

•

Boost output frequency

•

Temperature thresholds

•

Slope for brightness changes

•

Dithering options

•

PWM output resolution

•

Boost control bits

External components RISET and RFSET can also be used for selecting the output current and PWM frequencies.

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