DN440 - Versatile TFT LCD Bias Supply and White LED Driver in a 4mm × 4mm QFN

Versatile TFT LCD Bias Supply and White LED Driver
in a 4mm × 4mm QFN – Design Note 440
Eddy Wells
Introduction
The makers of handheld medical, industrial and consumer
devices use a wide variety of high resolution, small to
medium sized color TFT LCD displays. The power supply
designers for these displays must contend with shrinking board area, tight schedules, and variations in display
types and feature requirements. The LTC ®3524 simplifies
the designer’s job by combining a versatile, easily programmed, TFT LCD bias supply and white LED backlight
driver in a low profile 4mm × 4mm QFN package.
A second nonsynchronous boost converter generates the
voltage required to regulate one or two LED strings at
up to 25mA each. LED current can be adjusted by either
analog or digital means, optimizing the TFT display for
varying ambient light conditions. Each string is independently enabled and can contain 1 to 5 LEDs in series.
Internal circuitry maintains equal current in the strings,
even when the forward voltage drops of the LEDs do not
match. Open LED protection is provided to prevent the
output from exceeding 24V.
The LTC3524’s 2.5V to 6V input supply range is ideally
suited for portable devices powered from Li-Ion or
multiple alkaline or nickel cells. Both the LCD and LED
drivers operate at 1.5MHz, allowing the use of tiny, low
cost, inductors and capacitors.
3-Output TFT Supply with Digitally Dimmed LED
Backlight
A LTC3524-based TFT and backlight solution for a 4 to 6
inch LCD is shown in Figure 2. High frequency operation
of the power components and the QFN package shrinks
the total converter footprint to approximately 120mm2
(single sided).
The TFT bias portion of the circuit consists of a synchronous boost converter, adjustable between 3V and
6V, providing the main analog VOUT for the TFT. Low
current gate drive voltages (VH and VN) are generated
using integrated charge-pump circuits. These low noise
outputs are programmable to ±20V, allowing optimal bias
for multiple display types and makers. The TFT outputs
are sequenced at power-up and discharged at power-down
as shown in Figure 1.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
VH
VOUT
5V/DIV
VN
5ms/DIV
DN4EW F01
Figure 1. LTC3524 TFT LCD Supply Sequencing
at Power-Up and Power-Down
05/08/440
DN4EW F02
Figure 2. LTC3524-Based LCD and White LED Supply
The circuit schematic is shown in Figure 3. The TFT bias
portion of the circuit provides a 5V, 25mA output for the
TFT drivers as well as 12.5V and –7.5V outputs with up
to 2mA for the gate bias. These voltages are programmed
using the FBVO, FBH, and FBN pins respectively.
As shown in Figure 1, these outputs are sequenced with
VOUT, VN, then VH powered, as required by most displays.
The outputs are actively discharged when ELCD is brought
low, removing voltage from the display.
The white LED backlight for the Figure 3 circuit consists
of two strings with four series LEDs. The LEDs are
driven from the high side with the LTC3524, allowing the
strings to terminate at ground, reducing the number of
wires required to power the display. With RPROG = 100k,
each LED is regulated to 20mA. Maximum power for the
backlight is approximately 600mW, assuming a forward
voltage around 3.6V per element.
10μH
2.2μF
Li-Ion
5V
25mA
10μF
10V
10μF
SW2
VLED
VOUT
LED2
FBVO
VNIN
V2x
C2+
LED1
0.47μF
0.1μF
12.5V
2mA
0.47μF
PROG
ELED2
ELED1
ELCD
LTC3524
C2–
VH
220k
FBH
CH
+
VIN
470k
VN
–
CH GND
0.1μF
10V/DIV
LED2
500μs/DIV
CN+
DN4EW F04
Figure 4. Burst Dimming Waveforms
Efficiency results for this design are given in Figure 5
with a 3.6V input. The LCD efficiency curve shows the
performance of the synchronous boost converter with
VOUT at 5V and varying load current. This curve includes
the no load quiescent current of the charge-pumps, which
are powered from VOUT.
VOUT
–7.5V
2mA
VIN = 3.6V
90
100k for 20mA
1M
10V/DIV
12V
100
FBN
2M
12.5V
VLED
1M
324k
200mA/DIV
Analog dimming of the LEDs can be implemented by
adjusting the current through the PROG pin. Efficiency
for analog dimming is shown in Figure 5. Efficiency with
PWM dimming would remain close to 78% over a wide
dimming range.
3.3μH
VIN
SW1
5V/DIV
ILED BOOST
INDUCTOR
EFFICIENCY (%)
+
–
ELED1 AND
ELED2
LCD
80
LED
70
0.47μF
0.1μF
60
DN4EW F03
Figure 3. Complete TFT and LED Solution
Dimming is achieved by changing the duty cycle of a 200Hz
power signal applied to the LED strings. The frequency is
high enough to prevent visually detectable flickering, but
low enough to allow a better than 100:1 dimming range.
Dimming is implemented by simply connecting a microprocessor controlled port to ELED1 and ELED2. Scope
waveforms at 50% duty cycle are shown in Figure 4.
5
15
20
10
VOUT OR LED STRING CURRENT (mA)
25
DN4EW F05
Figure 5. LCD Bias and LED Efficiency
Conclusion
The LTC3524 shrinks and simplifies the design of small
to medium sized TFT LCDs by combining the LCD supply
and LED driver in a single compact package. LCD bias
voltages and LED currents are programmable, making
it possible to simplify parts stock by using the LTC3524
for a wide variety of displays.
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 3222
www.linear.com
Linear Technology Corporation
dn440f LT/TP 0508 246K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2008
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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