LP8550TLX-ASLASHNOPB datasheet(18/49 Pages) TI2

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OVP

Light

Load

OCP

Switch

Driver

Osc/

ramp

VREF

Boost output

voltage

adjustment

Active Load

Startup

LP8550

SNVS657E – SEPTEMBER 2010 – REVISED SEPTEMBER 2014

www.ti.com

8.3.3 Boost Converter

8.3.3.1 Operation

The LP8550 boost DC/DC converter generates a 10-V to 40-V supply voltage for the LEDs from 2.7-V to 22-V

input voltage. The output voltage can be controlled either with EEPROM register bits <VBOOST[4:0]> or

automatic adaptive voltage control can be used. The converter is a magnetic switching PWM mode DC/DC

converter with a current limit. The topology of the magnetic boost converter is called CPM (current programmed

mode) control, where the inductor current is measured and controlled with the feedback. Switching frequency is

selectable between 156 kHz and 1.25 MHz with EEPROM bit <BOOST_FREQ[1:0]>. When <EN_BOOST>

EEPROM register bit is set to 1, then boost activates automatically when backlight is enabled.

In adaptive mode the boost output voltage is adjusted automatically based on LED driver headroom voltage.

Boost output voltage control step size is in this case 125 mV to ensure as small as possible driver headroom and

high efficiency. Enabling the adaptive mode is done with <EN_ADAPT> EEPROM bit. If boost is started with

adaptive mode enabled, then the initial boost output voltage value is defined with the <VBOOST[4:0]> EEPROM

Figure 17 shows the boost topology with the protection circuitry:

Figure 17. Boost Topology with Protection Circuitry

8.3.3.2 Protection

Three different protection schemes are implemented:

1. Overvoltage protection, limits the maximum output voltage.

–

Overvoltage protection limit changes dynamically based on output voltage setting.

–

Keeps the output below breakdown voltage.

–

Prevents boost operation if battery voltage is much higher than desired output.

2. Overcurrent protection, limits the maximum inductor current.

3. Duty cycle limiting.

8.3.3.3 Manual Output Voltage Control

User can control the boost output voltage with <VBOOST[4:0]> EEPROM register bits when adaptive mode is

disabled.

VBOOST[4:0]

VOLTAGE (typical)

Bin

Dec

Volts

00000

00001

00010

00011

00100

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