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PDF LT1942 Data sheet ( Hoja de datos )
| Número de pieza | LT1942 | |
| Descripción | Quad DC/DC Converter | |
| Fabricantes | Linear | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LT1942 (archivo pdf) en la parte inferior de esta página. Total 16 Páginas | ||
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FEATURES
■ Triple Output TFT Supply with Built-In
Power Sequencing
■ Integrated White LED Backlight Driver
■ Integrated Schottky Diodes
■ Active Ballast Circuitry Ensures Precise Current
Matching in White LEDs
■ Low Noise 1MHz Fixed Frequency Operation
■ 2.6V to 16V Input Voltage Range
■ Soft-Start Limits Inrush Current
■ TFT Supply Output Voltages Independently Set
(Not Charge Pump Derived)
■ Power Good and Output Disconnect for TFT Supply
■ Built-In LED Dimming Capability
■ Open LED Protection for LED Driver
■ 24-Lead QFN Package (4mm × 4mm)
U
APPLICATIO S
■ Poly-Silicon TFT Displays
■ Amorphous Silicon TFT Displays
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
LT1942
Quad DC/DC Converter
for Triple Output TFT Supply
Plus LED Driver
DESCRIPTIO
The LT®1942 is a highly integrated quad switching regu-
lator designed to provide all necessary power supply
functions for TFT displays, including the white LED
backlight driver. The TFT supply incorporates two boost
and one inverting DC/DC converters. The TFT supply
output voltages are independently set, unlike charge
pump solutions which have many limitations. Power
sequencing for the TFT supply is built into the part and is
user programmable.
The LED driver is a boost DC/DC converter that can be
independently controlled. The LED driver has built-in
dimming control for precise control of LED current. An 8:1
dimming range is achieved by adjusting the CTRL4 volt-
age. The user can elect to drive a single string or two
strings of LEDs. An LED ballast circuit is included to
precisely match the LED currents if two strings of LEDs are
used. Soft-start is built into the LED driver as well as the
primary TFT supply.
The LT1942 is available in a low profile (0.75mm) 24-lead
QFN (4mm × 4mm) package.
TYPICAL APPLICATIO
TFT Bias and White LED Backlight Power Supply
VIN
3V TO 4.2V
2.2µF
VON 0.22µF
10V
2mA
10k
0.22µF
VOFF
–10V
2mA
665k
VIN
SHUTDOWN
LED CONTROL
47µH
47µH
22µH CMDSH-3
22µH
301k 4.7pF
698k D3 SW3 VCC SW1
FB3 FB1 100k 4.7µF
100k PGND14
PGND23
PGOOD
VOUT3
NFB2
LT1942
SW4
33µH
D4
VIN
D2
0.1µF
SW2
LED1
LED2
SHDN SGND/
CTRL4 AGND SS1
FB4
SS4 CT
0.1µF
0.1µF 4.99Ω
0.1µF
Si2301BDS
1M
AVDD
5V
40mA
4.7µF
20mA 20mA
1942 TA01
1942fb
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LT1942
FB4 Voltage
210
208
206
204
202
200
198
196
194
192
190
–40 –20
0 20 40 60
TEMPERATURE (°C)
80 100
1942 G04
Quiescent Current
10.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
–40 –20
0 20 40 60
TEMPERATURE (°C)
80 100
1942 G07
SW2 Saturation Voltage
400 TA = 25°C
350
300
250
200
150
100
50
0
0 10 20 30 40 50 60
SW2 CURRENT (mA)
1942 G10
VFB4 vs CTRL4
300
250
200
TA = –40°C
TA = 25°C
150
TA = 85°C
100
50
0
0 0.5 1 1.5 2
CTRL4 (V)
2.5 3
1942 G05
CT Timer Delay Time
10
9 CT = 0.1µF
8
7
6
5
4
3
2
1
0
–40 –20
0 20 40 60
TEMPERATURE (°C)
80 100
1942 G08
SW3 Saturation Voltage
400 TA = 25°C
350
300
250
200
150
100
50
0
0 10 20 30 40 50 60
SW3 CURRENT (mA)
1942 G10
Oscillator Frequency
1200
1100
1000
900
800
700
600
–40 –20
0 20 40 60
TEMPERATURE (°C)
80 100
1942 G06
SW1 Saturation Voltage
600 TA = 25°C
500
400
300
200
100
0
0 25 50 75 100 125 150 175 200
SW1 CURRENT (mA)
1942 G09
SW4 Saturation Voltage
800 TA = 25°C
700
600
500
400
300
200
100
0
0 100 200 300 400 500 600 700 800
SW4 CURRENT (mA)
1942 G12
1942fb
5
5 Page 
LT1942
APPLICATIO S I FOR ATIO
Setting the Output Voltages
Unlike charge-pump-based TFT supplies, the LT1942 TFT
outputs can all be independently set. The AVDD, VOFF and
VON outputs are all inductively based DC/DC converters.
As such, the output voltages are set by choosing the right
feedback divider ratio. The AVDD feedback pin is FB1 and
the reference voltage is 1.25V. The VOFF feedback pin is
NFB2 and the reference voltage is –150mV. The VON
feedback pin is FB3 and the reference voltage is 1.25V. The
LED driver is also an inductively based DC/DC converter.
Output Disconnect
The VON switching regulator has built in output discon-
nect. When the VON supply is enabled, the boost converter
will charge up the D3 node. With the output disconnect
PNP (Q5) turned off, the voltage on VOUT3 will be zero.
Once the power sequencing circuitry turns on the PNP,
VOUT3 will go to nearly the same voltage as D3.
The VOFF switching regulator naturally achieves output
disconnect due to the switching topology (inverting con-
figuration). When the LT1942 is in shutdown, VOFF will go
to zero volts.
The AVDD switching regulator provides a PGOOD pin to be
used for an external output disconnect function. This pin
can drive the gate of a PMOS device. When the LT1942 is
first enabled, the AVDD switching regulator begins to
charge up the output capacitor. Since PGOOD is still an
open collector at this point, the PMOS device is turned off,
leaving the AVDD output at zero volts. Once the output
capacitor charges to 97% of the final value, PGOOD
latches low turning on the PMOS device and bringing the
AVDD output quickly to the desired voltage.
Soft-Start
The higher current regulators, AVDD and LED, have user
programmable soft-start functions built into the part.
Simply place a capacitor from the SS1 pin to GND to
achieve soft-start for the AVDD switcher. For the LED
switcher, place a capacitor from the SS4 pin to GND. A
good value for both soft-start capacitors is 0.1µF. The
soft-start time is dependent on the load characteristics
which will vary depending on the type and size of the TFT
display. It is a good practice to experiment to determine
the best value for the soft-start capacitors.
Power Sequencing
The TFT supply has integrated supply sequencing built-in
to ensure proper initiation of the TFT display. Figure 1
shows a state diagram of the power sequencing circuitry.
The power sequencing circuitry insures that the AVDD
supply comes up first, then the negative VOFF supply and
lastly the VON positive supply. To pick the CT capacitor for
a particular delay, use the following equation:
CT
=
10µA • tDELAY
0.7V
Figure 2 shows the power sequencing during start-up of
the TFT power supply for the circuit shown in the figure on
the first page of this data sheet. All bias supplies start up
in a well controlled and well timed manner. The LED
backlight driver is independently controlled via the CTRL4
pin.
1942fb
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LT1942.PDF ] | |
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