LT1786F Datasheet, PDF (1/20 Pages) Linear Technology – SMBus Programmable CCFL Switching Regulator

LT1786F
SMBus Programmable
CCFL Switching Regulator
FEATURES
s _{Wide Battery Input Range: 4.5V to 30V}
s _{Grounded Lamp or Floating Lamp Configurations}
s _{Open Lamp Protection}
s _{Precision 100µA Full-Scale DAC}
Programming Current
s _{2-Wire SMBus Interface}
s _{Two Selectable SMBus Addresses}
s _{DAC Setting Is Retained in Shutdown}
U
APPLICATIONS
s _{Notebook and Palmtop Computers}
s _{Portable Instruments}
s _{Personal Digital Assistants}
DESCRIPTION
The LT ^®1786F is a fixed frequency, current mode, switch-
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 100µA full-
scale current output DAC. The DAC provides simple “bits-
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 40µA for standby
operation. The active low SHDN pin disables the CCFL
control circuitry, but keeps the DAC alive. Supply current
in this operating mode drops to 150µA.
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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATION
90% Efficient Floating CCFL with 2-Wire SMBus Control of Lamp Current
D1
BAT85
C7, 1µF
SHUTDOWN
1 _CCFL
PGND
2
I_CCFL
16
CCFL V_SW
15
BULB
3
DIO
14
BAT
4
LT1786F
13
CCFL V_C
ROYER
5
AGND
12
V_CC
6
SHDN
11
I_OUT
7
SMBSUS
10
SCL
8
ADR
9
SDA
CCFL BACKLIGHT APPLICATION CIRCUITS
CONTAINED IN THIS DATA SHEET ARE COVERED
BY U.S. PATENT NUMBER 5408162
AND OTHER PATENTS PENDING
C5
1000pF
R2
220k
LAMP
C2
27pF
10 6
3kV
3
2 14
+ C3B
2.2µF
35V
3V ≤ V_CC
+
≤ 6.5V
C4
2.2µF
C1*
0.068µF
R3
100k
Q2*
Q1*
L1
5
L1 = COILTRONICS CTX210605
L2 = COILTRONICS CTX100-4
*DO NOT SUBSTITUTE COMPONENTS
COILTRONICS [phone]
BAT
+ C3A ^{8V TO 28V}
2.2µF
35V
R1
750Ω
TO
SMBus
HOST
L2
100µH
D1
1N5818
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
0µA TO 50µA I_CCFL CURRENT GIVES
0mA TO 6mA LAMP CURRENT
FOR A TYPICAL DISPLAY.
1786F TA01
FOR ADDITIONAL CCFL/LCD CONTRAST APPLICATION CIRCUITS,
REFER TO THE LT1182/83/84/84F DATA SHEET
1