LP85571 datasheet(12/49 Pages) TI2

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Time

VBOOST

Driver

headroom

VBOOST

Light

Load

OCP

Osc/

ramp

VREF

VFB

Startup

OVP

LP8557, LP85571

SNVSA15B – DECEMBER 2013 – REVISED DECEMBER 2015

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8.3 Feature Description

8.3.1 Boost Converter Overview

8.3.1.1 Operation

The boost DC-DC converter generates a 7-V to 28-V boost output voltage from a 2.7-V to 5.5-V boost input

voltage.

The converter is a magnetic switching PWM mode DC-DC inductive boost converter with a current limit. It uses

current programmed mode control, where the inductor current is measured and controlled with the feedback.

During start-up, the soft-start function reduces the peak inductor current. Figure 20 shows the boost block

diagram.

Figure 20. Boost Circuit Block Diagram

8.3.1.2 Adaptive Boost Output Voltage Control

The boost converter operates in adaptive boost control mode. In this mode, the voltage at the LED pins is

monitored by the control loop. It raises the boost voltage when the measured voltage of ANY of the LED strings

falls below the voltage threshold of its corresponding LOW comparator. If the headrooms of ALL of the LED

strings are above the voltage threshold of their corresponding MID comparator, then the boost voltage is

lowered.

Figure 21. Adaptive Headroom Detail

8.3.2 Brightness Control

The brightness can be controlled using an external PWM signal or the Brightness registers accessible via an I2C

interface or both. Which of these two input sources is selected is set by the BRTMODE bits. The LP8557

operates exclusively in BRTMODE = 00. While the LP8557I, by default, operates in BRTMODE = 11, it can

operate in all BRTMODE settings by configuring the bits via the I2C interface. How the brightness is controlled in

each of the four possible modes is described in the following sections.

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