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PDF LT1186FC Data sheet ( Hoja de datos )
| Número de pieza | LT1186FC | |
| Descripción | DAC Programmable CCFL Switching Regulator(Bits-to-NitsTM) | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LT1186FC (archivo pdf) en la parte inferior de esta página. Total 16 Páginas | ||
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LT1186F
DAC Programmable
CCFL Switching Regulator
(Bits-to-NitsTM)
FEATURES
s Wide Battery Input Range: 4.5V to 30V
s Grounded Lamp or Floating Lamp Configurations
s Open Lamp Protection
s Precision 50µA Full-Scale DAC Programming Current
s Standard SPI Mode or Pulse Mode
s DAC Setting Is Retained in Shutdown
U
APPLICATIONS
s Notebook and Palmtop Computers
s Portable Instruments
s Retail Terminals
DESCRIPTION
The LT®1186F 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 8-bit 50µA full-
scale current output DAC. The DAC provides simple “bits-
to-lamp current control” and communicates in two inter-
face modes including standard SPI mode and pulse mode.
On power-up, the DAC counter resets to half-scale and the
DAC configures to SPI or pulse mode depending on the CS
signal level. In SPI mode, the system microprocessor
serially transfers the present 8-bit data and reads back the
previous 8-bit data. In pulse mode, the upper six bits of the
DAC configure as increment-only (1-wire interface) or
increment/decrement (2-wire interface) operation depend-
ing on the DIN signal level.
The LT1186F control circuitry operates from a logic supply
voltage of 3.3V or 5V. The IC also has a battery supply
voltage pin that operates from 4.5V to 30V. The LT1186F
draws 6mA typical quiescent current. An active low shut-
down pin reduces total supply current to 35µA for standby
operation and the DAC retains its last setting. A 200kHz
switching frequency minimizes magnetic component size.
Current mode switching techniques with cycle-by-cycle
limiting gives high reliability and simple loop frequency
compensation. The LT1186F is available in a 16-pin nar-
row SO package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Bits-to-Nits is a trademark of Linear Technology Corporation. 1 Nit = 1 Candela/meter2
TYPICAL APPLICATION
90% Efficient Floating CCFL with 1-Wire (Increment Only) Pulse Mode Control of Lamp Current
D1
BAT85
C7, 1µF
SHUTDOWN
FROM MPU
1 CCFL
PGND
2
ICCFL
16
CCFL VSW
15
BULB
3
DIO
14
BAT
4
LT1186F
13
CCFL VC
ROYER
5
AGND
12
VCC
6
SHDN
11
IOUT
7
CLK
10
DOUT
8 CS
DIN 9
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
UP TO 6mA
LAMP
10 6
C2
27pF
3kV
3 2 14
+ C3B
2.2µF
35V
VIN
+
3.3V
C4 OR 5V
2.2µF
C1*
0.068µF
R3
100k Q2*
Q1*
L1
5
L1 = COILTRONICS CTX210605
L2 = COILTRONICS CTX100-4
*DO NOT SUBSTITUTE COMPONENTS
COILTRONICS (407) 241-7876
BAT
+ C3A 8V TO 28V
2.2µF
35V
R1
750Ω
L2
100µH
D1
1N5818
ALUMINUM ELECTROLYTIC IS RECOMMENDED FOR C3A AND C3B.
MAKE 3CB ESR ≥ 0.5Ω TO PREVENT DAMAGE TO THE LT1186F HIGH-SIDE
SENSE RESISTOR DUE TO SURGE CURRENTS AT TURN-ON
C1 MUST BE A LOW LOSS CAPACITOR, C1 = WIMA MKP-20
Q1, Q2 = ZETEX ZTX849 OR ROHM 2SC5001
0µA TO 50µA ICCFL CURRENT GIVES
0mA TO 6mA LAMP CURRENT
FOR A TYPICAL DISPLAY.
LT1186F • TA01
FOR ADDITIONAL CCFL/LCD CONTRAST APPLICATION CIRCUITS,
REFER TO THE LT1182/83/84/84F DATA SHEET
1
1 page  
TYPICAL PERFORMANCE CHARACTERISTICS
ICCFL Summing Voltage
vs Temperature
0.53
0.52
0.51
0.50
0.49
0.48
0.47
0.46
0.45
0.44
0.43
0.42
0.41
0.40
0.39
0.38
–75 – 50 –25
0 25 50 75 100 125 150 175
TEMPERATURE (°C)
LT1186F • G07
∆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)
LT1186F • G10
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)
LT1186F • G13
ICCFL Summing Voltage
Load Regulation
5
4
3
2
1
0
–1
–2
–3
–4
–5
–6
–7
–8
–9
–10
0
T = –55°C
T = 25°C
T = 125°C
20 40 60 80 100 120 140 160 180 200
ICCFL PROGRAMMING CURRENT (µA)
LT1186F • G08
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)
LT1186F • G11
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)
LT1186F • G14
LT1186F
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)
LT1186F • G09
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)
LT1186F • G12
VC High Clamp Voltage
vs Temperature
2.4
2.3
2.2
2.1
2.0
1.9
1.8
1.7
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
LT1186F • G15
5
5 Page 
LT1186F
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 de-
vices 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 are responsible for almost 50% of the
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 LT1186F 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 LT1186F, 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 volt-
age or output load conditions. Finally, it allows simple
pulse-by-pulse current limiting to provide maximum switch
protection under output overload or short-circuit condi-
tions.
The LT1186F 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 35µA 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 LT1186F. The anti-sat
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LT1186FC.PDF ] | |
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