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PDF LT1786F Data sheet ( Hoja de datos )
| Número de pieza | LT1786F | |
| Descripción | SMBus Programmable CCFL Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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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
ICCFL
16
CCFL VSW
15
BULB
3
DIO
14
BAT
4
LT1786F
13
CCFL VC
ROYER
5
AGND
12
VCC
6
SHDN
11
IOUT
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
10 6
C2
27pF
3kV
3 2 14
+ C3B
2.2µF
35V
3V ≤ VCC
+ ≤ 6.5V
C4
2.2µF
R3
100k
C1*
0.068µF
Q2* Q1*
L1
5
L1 = COILTRONICS CTX210605
L2 = COILTRONICS CTX100-4
*DO NOT SUBSTITUTE COMPONENTS
COILTRONICS (561) 241-7876
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 ICCFL 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
1 page  
LT1786F
TYPICAL PERFORMANCE CHARACTERISTICS
VC Sink Offset Current
vs Temperature
10
9
8 CCFL VC = 1.5V
7
6
5 CCFL VC = 1.0V
4
3
2
1
0 CCFL VC = 0.5V
–1
–2
–3
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G10
Negative DIO Voltage
vs Temperature
1.6
1.4
I(DIO) = 10mA
1.2
1.0 I(DIO) = 5mA
0.8 I(DIO) = 1mA
0.6
0.4
0.2
0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G13
VC High Clamp Voltage
vs Temperature
2.4
2.3
∆CCFL VC Source Current for
∆ICCFL Programming Current
vs Temperature
5.10
5.05
5.00 ICCFL = 100µA
4.95 ICCFL = 50µA
ICCFL = 10µA
4.90
4.85
4.80
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G11
VC to DIO Current Servo Ratio
vs Temperature
103
102
101 I(DIO) = 10mA
100 I(DIO) = 1mA
99
I(DIO) = 5mA
98
97
96
95
–75 – 50–25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G14
VC Switching Threshold
vs Temperature
1.3
1.2
2.2 1.1
2.1 1.0
2.0 0.9
1.9 0.8
1.8 0.7
1.7
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G16
0.6
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G17
Positive DIO Voltage
vs Temperature
1.2
1.0
I(DIO) = 10mA
0.8 I(DIO) = 5mA
0.6
I(DIO) = 1mA
0.4
0.2
0
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G12
VC Low Clamp Voltage
vs Temperature
0.30
0.25
0.20
0.15
0.10
0.05
0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G15
BULB Protect Servo Voltage
vs Temperature
7.5
7.4
7.3
7.2
ICCFL = 100µA
7.1
7.0
6.9
ICCFL = 50µA
6.8
6.7
6.6 ICCFL = 10µA
6.5
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G18
5
5 Page 
WU W
TI I G DIAGRA S
Timing for SMBus Interface
LT1786F
tBUF
SDA
tHD:STA
tr
SCL
tLOW
STOP START
tHD:STA
tf
tHIGH
tHD:DAT
tSU:STA
tSU:DAT
START
tSU:STO
STOP
1786 TD01
Operating Sequence
SMBus Write Byte Protocol, with SMBus Address = 0101101B,
Command Byte = 0XXXXXXXB and Data Byte = 111111XXB
ADR
SMBUS ADDRESS
SDA 0 1 0 1 1 0 1
COMMAND BYTE
XXXXX X X
DATA BYTE
1 1 1 11 1X
X
VCC
GND
SCL
S
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
*
P
IOUT
S = START
P = STOP
* = OPTIONAL
1786 TD02
FULL-SCALE
CURRENT
ZERO-SCALE
CURRENT
APPLICATIONS INFORMATION
Introduction
Current generation portable computers and instruments
use backlit Liquid Crystal Displays (LCDs). Cold Cathode
Fluorescent Lamps (CCFLs) provide the highest available
efficiency in back lighting the display. Providing the most
light out for the least amount of input power is the most
important goal. These lamps require high voltage AC to
operate, mandating an efficient high voltage DC/AC con-
verter. The lamps operate from DC, but migration effects
damage the lamp and shorten its lifetime. Lamp drive
should contain zero DC component. In addition to good
efficiency, the converter should deliver the lamp drive in
the form of a sine wave. This minimizes EMI and RF
emissions. Such emissions can interfere with other
devices and can also degrade overall operating efficiency.
Sinusoidal CCFL drive maximizes current-to-light conver-
sion in the lamp. The circuit should also permit lamp
intensity control from zero to full brightness with no
hysteresis or “pop-on.”
The small size and battery-powered operation associated
with LCD equipped apparatus dictate low component
count and high efficiency for these circuits. Size con-
straints place severe limitations on circuit architecture and
long battery life is a priority. Laptop and handheld portable
computers offer an excellent example. The CCFL and its
power supply can be responsible for almost 50% of the
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LT1786F.PDF ] | |
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