LP8555 datasheet(46/64 Pages) TI1

46 / 64 page

LP8555

SW_A

FB_A

VLDO

GNDs

EN/VDDIO

ISET/SCL

PWM/INT

LEDA1

LEDA2

LEDA3

LEDA4

LEDB3

LEDB4

2.7V ± 4.5V

CVLDO

VDD

VBATT

FSET/SDA

LEDB5

LEDB6

SW_B

FB_B

2 x 4.7 2F

VBATT

LEDA5

LEDA6

LEDB1

LEDB2

VOUT_A

7 - 28V

VOUT_B

CVDD

SDA

SCL

RPULL-UP

PWM

Brightness Control

PWM input only

4.7 ± 6.8µH

1µF

10µF

4.7 ± 6.8µH

2.7V ± 4.5V

LED

Current up

to 25mA /

string

LED

Current up

to 25mA /

string

7 - 28V

(Single Li-Ion Cell)

LCD Display

COUT_B

CIN_B

COUT_A

CIN_A

2 x 4.7 2F

10µF

39 pF

LP8555

SNVS857 – FEBRUARY 2014

www.ti.com

9 Application and Implementation

9.1 Application Information

The LP8555 designed for LCD backlighting, especially for high-resolution tablet panels where more backlight

power is needed due to smaller aperture ratio of the LCD. With single-boost configuration the inductor selection

is difficult for height restricted applications; to overcome this LP8555 uses dual-boost configuration. This shares

the total load to two boost inductors and allows using two smaller inductors instead of one large inductor while

maintaining good efficiency. 12 LED current sinks allow driving up to 96 LEDs with high efficiency. Better

efficiency is achieved with using lower conversion ratio for boost and driving more LEDs in parallel, compared to

using fewer LED strings and higher boost conversion ratio. Main limiting factor for output power is inductor

current limit, which is calculated in the Detailed Design Procedure. PCB thermal performance must be

considered in high power applications where thermal dissipation of LP8555 can become limiting factor.

Due to a flexible input voltage configuration, the LP8555 can be used also in various other applications, such as

laptop backlighting, as well as other LED lighting where high number of LEDs are needed and must be driven

with highest possible efficiency. The following design procedure can be used to select component values for the

LP8555. An example for default EPROM configuration is given with corresponding design parameters.

LP8555EVM User Guide has reference bill of materials and example layout pictures.

9.2 Typical Applications

9.2.1 Application for Default LP8555YFQR EPROM Configuration

With the default EPROM configuration PWM input is used for brightness control. The backlight is

enabled/disabled and also configuration can be changed before backlight turning on with I2C writes. Up to 12

LED strings can be used with max 28-V boost output voltage. LED current is set to 25 mA by default. See

detailed EPROM setup in LP8555YFQR EPROM Configuration.

9.2.1.1 Schematic

Figure 43. Application Diagram for Default EPROM Setup

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LP8555 Datasheet(PDF) 46 Page - Texas Instruments

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