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PDF LT1768 Data sheet ( Hoja de datos )
| Número de pieza | LT1768 | |
| Descripción | High Power CCFL Controller | |
| Fabricantes | Linear | |
| Logotipo |  |
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FEATURES
s Ultrawide Multimode DimmingTM Range
s Multiple Lamp Capability
s Programmable PWM Dimming Range and
Frequency
s Precision Maximum and Minimum Lamp
Currents Maximize Lamp Lifetime
s No Lamp Flicker Under All Supply and Load
Conditions
s Open Lamp Detection and Protection
s 350kHz Switching Frequency
s 1.5A MOSFET Gate Driver
s 100mV Current Sense Threshold
s 5V Reference Voltage Output
s The 16-Lead SSOUP Package
APPLICATIO S
s Desktop Flat Panel Displays
s Multiple Lamp Displays
s Notebook LCD Displays
s Point of Sale Terminal Displays
LT1768
High Power CCFL Controller
for Wide Dimming Range and
Maximum Lamp Lifetime
DESCRIPTIO
The LT®1768 is designed to control single or multiple cold
cathode fluorescent lamp (CCFL) displays. A unique Mul-
timode Dimming scheme* combines both linear and PWM
control functions to maximize lamp life, efficiency, and
dimming range. Accurate maximum and minimum lamp
currents can be easily set. The LT1768 can detect and
protect against lamp failures and overvoltage start-up
conditions. It is designed to provide maximum flexibility
with a minimum number of external components.
The LT1768 is a current mode PWM controller with a 1.5A
MOSFET driver for high power applications. It contains a
350kHz oscillator, 5V reference, and a current sense
comparator with a 100mV threshold. It operates from an
8V to 24V input voltage. The LT1768 also has undervoltage
lockout, thermal limit, and a shutdown pin that reduces
supply current to 65µA. It is available in a small 16-lead
SSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Multimode Dimming is a trademark of Linear Technology Corporation.
*Patent Pending
TYPICAL APPLICATIO
C4-WIMA MKP2
L1-COILTRONICS UP4-680
T1-2 CTX110607 IN PARALLEL
Q1-ZDT1048
*R5 CAN BE METAL PCB TRACE
LAMP
PGND GATE
33pF
LAMP
VIN
8V – 24V
C1
33µF
33pF T1
4
6 10
53
2
C4
0.33µF
1
PROG
0V TO 5V OR
1kHz PWM
C2
0.033µF
C3
0.1µF
DI02
DI01
VIN
VREF
SENSE FAULT
LT1768
VC SHDN
AGND
CT
RMIN
RMAX
R4
16.2k
PROG PWM
R1
49.9k
C4
10µF
5V
0.1µF
250Ω
1/4W
Q1
R2
40.2k
R3
60.4k
MBRS130T3
100
2200pF
Q1
L1
68µH
Si3456DY
R5*
0.025
1768 TA01
Figure 1. 14W CCFL Supply Produces a 100:1 Dimming Ratio While
Maintaining Minimum and Maximum Lamp Current Specifications
Lamp Output and Dimming
Ratio vs Lamp Current
10000
1000
DIMMING RATIO (NITS/NITS)
100
LAMP OUTPUT (NITS)
10
1
0.1
0
LAMP MANUFACTURERS
SPECIFIED CURRENT RANGE
2 4 68
LAMP CURRENT (mA)
10
1768 TA01b
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LT1768
Switching Frequency vs Temperature
400
390
380
370
360
350
340
330
320
310
300
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
1768 G10
FAULT Pin Saturation Voltage vs
Current
450
IDIO1 = 0µA
400 IDIO2 = 0µA
350
300
250
200
150
100
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
IFAULT (mA)
1768 G13
DIO Pin Voltage vs Current
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0 2 4 6 8 10 12 14 16 18 20
DIO CURRENT (mA)
1768 G24
PWM Frequency vs Temperature
124
120
CT = 0.22µF
VPWM = 2.5V
116
112
108
104
100
96
92
88
84
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
1768 G11
Sense Pin Bias Current vs
Temperature
50
45 VSENSE = 0V
40
35
30
25
20
15
10
5
0
–50 –25
0 25 50 75 100 125
TEMPERATURE (°C)
1768 G14
DIO Pin Voltage vs Current
– 2.0
–1.8
–1.6
–1.4
–1.2
–1.0
– 0.8
– 0.6
– 0.4
– 0.2
0
0 – 2 – 4 – 6 – 8 –10 –12 –14 –16 –18 –20
DIO CURRENT (mA)
1768 G20
FAULT Pin Saturation Voltage vs
Temperature
0.250
0.225
0.200
0.175
0.150
0.125
0.100
0.75
0.50
0.25
0
–50 –25
IDIO1 = 0µA
IDIO2 = 0µA
IFAULT = 1mA
0 25 50 75 100 125
TEMPERATURE (°C)
1768 G12
Maximum Gate Voltage vs
Temperature
15.00 IGATE = –10mA
14.50
14.00
13.50
13.00
VIN = 24V
12.50
12.00
11.50
11.00
10.50
VIN = 12V
10.00
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1768 G15
VC Clamp Voltage vs Current
3.75
3.74
3.73
3.72
3.71
3.70
3.69
3.68
3.67
3.66
3.65
0
50 100 150 200 250 300 350 400 450 500
VC CURRENT (µA)
1768 G25
5
5 Page 
LT1768
APPLICATIONS INFORMATION
per lamp to 1mA for all PROG voltages but limits the
dimming ratio to 6:1.
Trace B in Figures 3a and 3b shows Figure 1’s CCFL
current waveform operating at 1mA in PWM mode.
Maximum current mode (VPROG > 4V) sets the VC source
current to five times the current out of the 1.25V refer-
enced RMAX pin. Setting RRMAX to produce CCFL current
equal to the manufacturer’s maximum rating in this mode
insures no degradation in the specified lamp lifetime. For
example, setting R4 in the circuit in Figure 1 to 16.2k sets
the maximum CCFL current to 9mA (9mA is only a typical
maximum lamp current used for illustration, consult lamp
specifications for the actual value). Trace A in Figure 3a
and 3b shows Figure 1’s CCFL current waveform operating
at 9mA in maximum current mode.
TRACE A
VPROG = 4.5V
ILAMP = 9mARMS
TRACE B
VPROG = 1.125V
ILAMP = 1mARMS
1ms/DIV
Figure 3a. CCFL Current for Circuit in Figure 1
TRACE A
VPROG = 4.5V
ILAMP = 9mARMS
TRACE B
VPROG = 1.125V
ILAMP = 1mARMS
100µs/DIV
Figure 3b. CCFL Current for Circuit in Figure 1
In linear mode (VPWM < VPROG < 4V), VC source current is
controlled linearly with the voltage on the PROG pin. The
equation for the VC source current in linear mode is
IVC = (VPROG – 1V)/3V (IRMAX • 5). For the best current to
light conversion and highest efficiency, VPWM should be
set to make the LT1768 normally operate in the linear
mode. For example, in the circuit in Figure 1, linear mode
runs from VPROG = 3V to VPROG = 4V with lamp current
equal to (3mA)(VPROG–1V)/1V.
In PWM Mode (1V < VPROG < VPWM), the VC source current
is modulated between the value set by minimum current
mode and the value for IVC in linear mode with VPROG =
VPWM. The PWM frequency is equal to 22Hz/CT(µF) with
its duty cycle set by the voltages on the PROG and PWM
pins and follows the equation:
DC = [1 – (VPWM – VPROG)/(VPWM – 1V)] • 100%
The LT1768’s PWM mode enables wide dimming ratios
while reducing the high crest factor found in PWM only
dimming solutions. In the example of Figure 1, PWM
mode runs from VPROG = 1V to VPROG = 3V with CCFL
current modulated between 0mA and 6mA. The PWM
modulation frequency is set to 220Hz by capacitor C3.
When combined, these five modes of operation allow
creation of a DC controlled CCFL current profile that can be
tailored to each particular display. With linear mode CCFL
current control over the most widely used current range,
and PWM mode at the low end, the LT1768 enables wide
dimming ratios while maximizing CCFL lifetimes.
Lamp Feedback Current
In a typical application, the DIO1/2 pins are connected to
the low voltage side of the lamps. Each DIO pin is the
common connection between the cathode and anode of
two internal diodes (see Block Diagram). The remaining
terminals of the diodes are connected to PGND. Bidirec-
tional lamp current flows into the DIO1/2 pins and their
diodes conduct alternately on the half cycles. The diode
that conducts on the negative cycle has a percentage of its
current diverted into the VC pin. This current nulls against
the VC source current specified by the Multimode Dim-
ming section. A single capacitor on the VC pin provides
both stable loop compensation and an averaging function
to the halfwave-rectified lamp current. Therefore, current
into the VC pin from the lamp current programming
section relates to average lamp current.
The overall gain from the resistor current to average lamp
current is equal to the gain from the Multimode Dimming
block divided by the gain from the DIO pin to the VC pin,
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LT1768.PDF ] | |
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