1

LT1786F

FEATURES

s

Wide Battery Input Range: 4.5V to 30V

s

Grounded Lamp or Floating Lamp Configurations

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Open Lamp Protection

s

Precision 100

µA Full-Scale DAC

Programming Current

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2-Wire SMBus Interface

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Two Selectable SMBus Addresses

s

DAC Setting Is Retained in Shutdown

to-lamp-current control” and communicates using the

2-wire SMBus serial interface. The LT1786F acts as an

SMBus slave device using one of two selectable SMBus

addresses set by the address pin ADR.

On Power-up, the DAC output current assumes midrange

or zero scale, depending on the logic state of the ADR

pin.The entire IC can be shut down through the SMBSUS

pin or by setting the SHDN bit = 1 in the SMBus command

byte. Digital data for the DAC output current is retained

internally and the supply current drops to 40

µAforstandby

operation. The active low SHDN pin disables the CCFL

control circuitry, but keeps the DAC alive. Supply current

in this operating mode drops to 150

µA.

The LT1786F control circuitry operates from a logic supply

voltage of 3.3V or 5V. The IC also has a battery supply pin

that operates from 4.5V to 30V. The LT1786F draws 6mA

typical quiescent current. A 200kHz switching frequency

minimizes magnetic component size. Current mode switch-

ing techniques with cycle-by-cycle limiting gives high

reliability and simple loop frequency compensation. The

LT1786F is available in a 16-pin narrow SO package.

The LT

ing regulator that provides the control function for Cold

Cathode Fluorescent Lighting (CCFL). The IC includes an

efficient high current switch, an oscillator, output drive

logic, control circuitry and a micropower 6-bit 100

µA full-

scale current output DAC. The DAC provides simple “bits-

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Notebook and Palmtop Computers

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Portable Instruments

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Personal Digital Assistants

90% Efficient Floating CCFL with 2-Wire SMBus Control of Lamp Current

BAT

8V TO 28V

16

15

14

13

12

11

10

9

DIO

AGND

SHDN

SMBSUS

ADR

BULB

BAT

ROYER

SCL

SDA

CCFL

PGND

LT1786F

LAMP

10

6

L1

R1

750

Ω

L2

100

µH

3

2

1

5

4

+

+

C7, 1

µF

C1*

0.068

µF

C5

1000pF

R2

220k

R3

100k

C3A

2.2

µF

35V

C4

2.2

µF

3V

≤ 6.5V

TO

SMBus

HOST

1786F TA01

C3B

2.2

µF

35V

C2

27pF

3kV

+

Q2*

Q1*

SHUTDOWN

ALUMINUM ELECTROLYTIC IS RECOMMENDED FOR C3A AND C3B.

MAKE 3CB ESR

≥ 0.5Ω TO PREVENT DAMAGE TO THE LT1786F HIGH-SIDE

SENSE RESISTOR DUE TO SURGE CURRENTS AT TURN-ON

C1 MUST BE A LOW LOSS CAPACITOR, C1 = WIMA MKI OR MKP-20

= PANASONIC ECH-U

Q1, Q2 = ZETEX ZTX849 OR ROHM 2SC5001

0

0mA TO 6mA LAMP CURRENT

FOR A TYPICAL DISPLAY.

FOR ADDITIONAL CCFL/LCD CONTRAST APPLICATION CIRCUITS,

REFER TO THE LT1182/83/84/84F DATA SHEET

D1

BAT85

1

2

3

4

5

6

7

8

D1

1N5818

L1 = COILTRONICS CTX210605

L2 = COILTRONICS CTX100-4

*DO NOT SUBSTITUTE COMPONENTS

COILTRONICS (561) 241-7876

CCFL BACKLIGHT APPLICATION CIRCUITS

CONTAINED IN THIS DATA SHEET ARE COVERED

BY U.S. PATENT NUMBER 5408162

AND OTHER PATENTS PENDING

SMBus Programmable

CCFL Switching Regulator

, LTC and LT are registered trademarks of Linear Technology Corporation.