OVP

Light

Load

SW

R

R

OCP

Switch

Driver

R

S

R

$

+

-

+

-

Osc/

ramp

-

+

VREF

FB

Boost output

voltage

adjustment

Active Load

Startup

GD

LP8545

SNVS635D – APRIL 2010 – REVISED DECEMBER 2013

www.ti.com

Boost Converter

Operation

The LP8545 boost DC/DC converter generates a 10…40V supply voltage for the LEDs from 2.7…22V input

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

adaptive voltage control can be used. Higher output voltages can be achieved with external FET and by using

resistor divider in the FB pin. GD pin operates as gate driver for the external FET in this case. To activate

external FET gate driver, <EN_EXT_FET> bit in EEPROM register must be set to 1. 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 will activate

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

register bits in order to eliminate long output voltage iteration time when boost is started for the first time. The

following figure shows the boost topology with the protection circuitry:

Protection

Three different protection schemes are implemented:

1. Over-voltage protection, limits the maximum output voltage.

–

Over-voltage 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. Over-current protection, limits the maximum inductor current.

3. Duty cycle limiting.

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

0

10

00001

1

11

00010

2

12

00011

3

13

20

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