November 2004 - Compact Power Supply Drives TFT-LCD and LED Backlight

DESIGN IDEAS
Compact Power Supply Drives
TFT-LCD and LED Backlight by Dongyan Zhou
Introduction
Li-Ion to 4-Inch or
The LT1942 is a highly integrated, 5-Inch TFT-LCD
L1
22µH
VIN
3V TO 4.2V
R5
442k
C3
0.22µF
16V
C2
0.22µF
16V
R6
63.4k
R4
10k
R3
665k
VIN
D3
FB3
SW3
0.1µF 16V
L5 47µH
L2 47µH
SHUTDOWN
C6
SW1
FB1
NFB2
SW4
LT1942
D4
D2
LED1
SW2
LED2
SHDN
CTRL4 AGND
LED CONTROL
PGND14
PGND23
SS1
SS4
C7
0.1µF
C8
0.1µF
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
5
R2
100k
L4 33µH
20
10
15
LED2 CURRENT (mA)
Figure 2. Typical current matching
between LED1 and LED2
4.7pF
R8
1M
AVDD
5V
40mA
C1
4.7µF
6.3V
PGOOD
PGND23
0.8
0
R1
301k
VCC
0.9
M1
PMOS
PGND14
VOUT3
1.0
D1
L3 22µH
C5
2.2µF
VON
10V
2mA
VOFF
–10V
2mA
Figure 1 shows a complete power supply for three TFT bias voltages (AVDD,
VON, and VOFF) and a white LED driver.
A typical application of this design
is a 4- or 5- inch amorphous silicon
TFT-LCD panel powered by a single
cell Li-ion input. Two boost converters are used to supply AVDD and VON,
while the negative output converter
generates VOFF.
The LT1942 has built-in power
sequencing to properly power up the
TFT panel. When the shutdown pin is
driven above 1V, the AVDD switcher is
enabled first. After its output reaches
97% of the set value, the PGOOD pin
is driven low, which enables both the
VOFF and VON switchers. A built-in PNP
separates the VON bias supply from its
boost regulator output. The PNP is not
turned on until the programmable delay set by the CT pin has elapsed. The
panel is not activated and stays in a
low current state until VON is present.
This delay gives the column drivers
and the digital part of the LCD panel
time to get ready before the panel is
turned on.
LED1 CURRENT MATCHING ERROR (%)
4-output switching regulator designed
to power small to medium size TFT
panels. Three of the switching regulators provide the TFT bias voltages. The
fourth regulator is designed to drive
backlight LEDs.
The TFT supply includes two boost
converters and one negative output
DC/DC converter. Since different types
of panels may require different bias
voltages, all three output voltages are
adjustable for maximum flexibility.
The LED driver is a boost converter
that has built-in precise dimming
control. The user can choose to drive
a single string or two strings of LEDs.
A built-in ballast circuit helps to
match the LED currents precisely if
two strings are used.
All four regulators are synchronized
to a 1MHz internal clock, allowing the
use of small, low cost inductors and
ceramic capacitors. Programmable
soft-start capability is available for
both the primary TFT supply and LED
driver to control the inrush current.
The LT1942 is available in a tiny 4mm
× 4mm QFN package.
The fourth switcher in the LT1942
is a boost regulator designed to drive
up to 20 LEDs (in two strings) to power
the backlight. Built-in current ballast
circuitry keeps the current into LED1
and LED2 actively matched, regardless
of the difference in the LED voltage
drops. Figure 2 demonstrates the
current matching between the two
LED strings. The LED regulator has
a control pin (CTRL4), which provides
both shutdown and dimming functions. If any LED fails open, the output
of the LED regulator (D4) is clamped
VIN
20mA
20mA
C4
4.7µF
25V
FB4
CT
C9
0.1µF
R7
4.99
C1 TO C9: X5R OR X7R
D1: ZHCS400 ZETEX SEMICONDUCTOR
L1: 22µH MURATA LQH32CN220K53
L3: 22µH TAIYO YUDEN LB2012T220M
L2, L5: 47µH TAIYO YUDEN LB2012T470M
L4: 33µH SUMIDA CDPH4D19-330MC
M1: Si2301BDS SILICONIX
Figure 1. TFT bias voltages and LED backlight power supply from single Lithium-Ion battery input
Linear Technology Magazine • November 2004
31
DESIGN IDEAS
at around 42V to protect the internal
power devices.
Proper layout is important to achieve
the best performance. Paths that
carry high switching current should
be kept short and wide to minimize
the parasitic inductance. In the boost
regulator, the switching loop includes
the internal power switch, the Schottky
diode (internal or external), and the
output capacitor. In the negative
output regulator, the switching loop
includes the internal power switch,
the flying capacitor between the SW2
and D2 pins, and the internal Schottky
diode.
Connect the output capacitors of
the AVDD and LED switchers directly
to the PGND14 pin before returning to
the ground plane. Connect the output
capacitor of the VON switcher to the
PGND23 pin before returning to the
ground plane. Also connect the bottom
feedback resistors to the AGND pin.
Connect the PGND14, PGND23 and
AGND pins to the top layer ground
pad underneath the exposed copper
ground on the backside of the IC.
The exposed copper helps to reduce
thermal resistance. Multiple vias into
ground layers can be placed on the
ground pad directly underneath the
part to conduct the heat away from
the part.
LTC3426, continued from page 22
Component Selection
current should be greater than 1A.
A low forward voltage Schottky diode
reduces power loss in the converter
circuit.
Layout Considerations
least 750mA from a VIN as low as 3V.
When fully charged to 4.2V, over 1A
can be supplied. The photograph of
a demonstration board in Figure 5
shows just how small the board area
is for this application, 10mm × 12mm.
Tiny ceramic bypass capacitors and
surface mount inductors keep the
design small.
Figure 6 shows efficiency exceeding
90% and remaining greater than 85%
over a load range from 10mA to 900mA
with a fully charged battery.
LTC3426
SHDN
FB
R1
95.3k
1%
R2
30.9k
1%
VOUT
5V
750mA AT 3V
C2
22µF
80
VIN = 3V
75
70
65
60
50
Figure 4. Compact application circuit for VOUT at 5V
further eases the burden of heavy
capacitive loads by providing strong
pull-up currents during rising edges to
reduce the rise time. Thanks to these
two features, the LTC4302 enables the
implementation of much larger 2-wire
bus systems than are possible with a
simple unbuffered multiplexer.
VIN = 4.2V
85
55
C1: TDK C1608X5R0J475M
C2: TAIYO YUDEN JMK316BJ226ML
D1: ON SEMICONDUCTOR MBR120VLSFT1
L1: SUMIDA CDRH4D28-2R2 2
LTC4302, continued from page 26
EFFICIENCY (%)
VOUT
GND
32
100
90
SW
OFF ON
The addition of the LTC3426 to Linear
Technology’s high performance boost
converter family allows the designer
to deliver high current levels with
minimal board space. An on chip
switch and internal loop compensation
reduces component count to provide
an inexpensive solution for spot regulation applications.
D1
VIN
C1
10µF
Conclusion
95
L1
2.2µH
VIN
3V TO 4.2V
The LTC3426 requires just a few external components to accomodate various
VIN and VOUT combinations. Selecting
the proper inductor is important to
optimize converter performance and
efficiency. An inductor with low DCR
increases efficiency and reduces selfheating. Since the inductor conducts
the DC output current plus half the
peak-to-peak switching current, select
an inductor with a minimum DC rating of 2A. To minimize VOUT ripple,
use low ESR X5R ceramic capacitors.
The average Schottky diode forward
current is equal to the DC output
current therefore the diode average
Figure 5. Photograph of demo
board of circuit in Figure
4—board area is 10mm × 12mm
For further information on any
of the devices mentioned in this
issue of Linear Technology, use
the reader service card or call the
LTC literature service number:
1-800-4-LINEAR
Ask for the pertinent data sheets
and Application Notes.
1
10
100
LOAD CURRENT (mA)
1000
Figure 6. Up to 92% efficiency in Lithium-Ion
battery to 5V output applications
Impedance Analyzer, continued from page 30
assume that either the inductance is
well damped, or it is shunted by large
value capacitances.
Notes
1. This subject is treated in some detail in the
LTC1647 data sheet, Figures 9, 10, and 11
inclusive.
2. An hp 5210A Frequency Meter or any common
counter gives adequate accuracy for most measurements.
Linear Technology Magazine • November 2004