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LT1786F Schematic ( PDF Datasheet ) - Linear Technology

Teilenummer LT1786F
Beschreibung SMBus Programmable CCFL Switching Regulator
Hersteller Linear Technology
Logo Linear Technology Logo 




Gesamt 20 Seiten
LT1786F Datasheet, Funktion
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
750
TO
SMBus
HOST
L2
100µH
D1
1N5818
ALUMINUM ELECTROLYTIC IS RECOMMENDED FOR C3A AND C3B.
MAKE 3CB ESR 0.5TO 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






LT1786F Datasheet, Funktion
LT1786F
TYPICAL PERFORMANCE CHARACTERISTICS
BULB Input Bias Current
vs Temperature
10
8
6
4
2
0
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1787 G19
High-Side Sense Supply Current
vs Temperature
150
140
130
120
110
100
90
80
70
60
50
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G20
High-Side Sense Null Current
vs Temperature
1.060
1.040
1.020
1.000
0.980
0.960
0.940
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G21
High-Side Sense Null Current Line
Regulation vs Temperature
0.160
0.140
0.120
0.100
0.080
0.060
0.040
0.020
0.000
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G22
VSW Sat Voltage
vs Switch Current
1.0
0.9
0.8
T = 25°C
0.7
T = 125°C
T = –5°C
0.6
0.5
0.4
0.3
0.2
0.1
0
0 0.3 0.6 0.9 1.2 1.5
SWITCH CURRENT (A)
1787 G23
VSW Current Limit vs Duty Cycle
2.5
2.0
T = 0°C
T = 25°C
1.5
T = 125°C
1.0 MINIMUM
0.5
0
0 10 20 30 40 50 60 70 80 90
DUTY CYCLE (%)
1786 G24
Forced Beta vs ISW on VSW
110
100
90
80
70
60
50
40
30
20
10
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
CCFL ISW (A)
1786 G25
Full-Scale Output Current
vs Temperature
104
V(IOUT) = 0.465V
103
102
101
100
99
98
97
96
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G26
6

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LT1786F pdf, datenblatt
LT1786F
APPLICATIONS INFORMATION
battery drain. Additionally, all components, including PC
board and hardware, usually must fit within the LCD
enclosure with a height restriction of 5mm to 10mm.
The CCFL regulator drives an inductor that acts as a
switched-mode current source for a current-driven Royer-
class converter with efficiencies as high as 90%. The
control loop forces the CCFL PWM to modulate the aver-
age inductor current to maintain constant current in the
lamp. The constant current value, and thus lamp intensity,
is programmable. This drive technique provides a wide
range of intensity control. A unique lamp-current pro-
gramming block permits either grounded lamp or floating
lamp configurations. Grounded lamp circuits directly sense
one-half of average lamp current. Floating lamp circuits
directly sense the Royer’s primary-side converter current.
Floating-lamp circuits provide symmetric differential drive
to the lamp and reduce the parasitic loss from stray lamp-
to-frame capacitance, extending illumination range.
Block Diagram Operation
The LT1786F is a fixed frequency, current mode switching
regulator. A fixed frequency, current mode switcher con-
trols switch duty cycle directly by switch current rather
than by output voltage. Referring to the block diagram for
the LT1786F, the switch turns ON at the start of each
oscillator cycle. The switch turns OFF when switch current
reaches a predetermined level. The control of output lamp
current is obtained by using the output of a unique
programming block to set current trip level. The current
mode switching technique has several advantages. First,
it provides excellent rejection of input voltage variations.
Second, it reduces the 90° phase shift at mid-frequencies
in the energy storage inductor. This simplifies closed-loop
frequency compensation under widely varying input
voltage or output load conditions. Finally, it allows simple
pulse-by-pulse current limiting to provide maximum
switch protection under output overload or short-circuit
conditions.
The LT1786F incorporates a low dropout internal regula-
tor that provides a 2.4V supply for most of the internal
circuitry. This low dropout design allows input voltage to
vary from 3V to 6.5V with little change in quiescent
current. An active low shutdown pin typically reduces total
supply current to 150µA by shutting off the 2.4V regulator
and locks out switching action for standby operation. The
IC incorporates undervoltage lockout by sensing regulator
dropout and locking out switching below about 2.5V. The
regulator also provides thermal shutdown protection that
locks out switching in the presence of excessive junction
temperatures.
A 200kHz oscillator is the basic clock for all internal timing.
The oscillator turns on the output switch via its own logic
and driver circuitry. Adaptive anti-sat circuitry detects the
onset of saturation in the power switch and adjusts base
drive current instantaneously to limit switch saturation.
This minimizes driver dissipation and provides rapid turn-
off of the switch. The CCFL power switch is guaranteed to
provide a minimum of 1.25A in the LT1786F. The antisat
circuitry provides a ratio of switch current to driver current
of about 50:1.
Digital Interface
The LT1786F communicates with an SMBus host using
the standard 2-wire SMBus interface. The Timing Diagram
shows the signals on the SMBus. The two bus lines SDA
and SCL must be high when the bus is not in use. External
pull-up resistors or current sources are required at these
lines.
The LT1786F is a receive-only (slave) device. The master
must apply the following Write Byte protocol to commu-
nicate with the LT1786F:
17
1 1 8 1 8 11
S Slave Address WR A Command Byte A Data Byte A P
S = Start Conditon, WR = Write Bit, A = Acknowledge Bit, P = Stop Condition
The master initiates communication with the LT1786F
with a START condition (see SMBus Operating Sequence)
and a 7-bit address followed by the write bit = 0. The
LT1786F acknowledges and the master delivers the
command byte. The LT1786F acknowledges and latches
the active bits of the command byte into register A (see
Block Diagram) at the falling edge of the acknowledge
pulse. The master sends the data byte and the LT1786F
acknowledges the data byte. The data byte is latched into
register C at the falling edge of the final acknowledge pulse
and the DAC current output assumes the new 6-bit data
12

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