Nov 1999 Cost and Space Efficient Backlighting for Small LCD Panels

DESIGN IDEAS
Cost and Space Efficient Backlighting
for Small LCD Panels
by Jim Williams
A generation of small, portable,
“palmtop” computing devices has
recently appeared. These products
have small LCD displays that use
cold cathode fluorescent lamps
(CCFLs) for backlighting. These lamps
require high voltage AC current drive.
Circuitry for this purpose should be
physically small, cost effective and
electrically efficient.
Figure 1 shows a design that meets
the above criteria. The configuration
is a current-fed resonant Royer
converter driven by an LT1317B
micropower switching regulator. The
LT1317B effects a switch-mode cur-
rent sink, supplying the required
Royer drive to close a loop at the FB
pin. This path includes the lamp and
a filter network that rectifies T1’s
high voltage AC output into DC. In
this case, the circuit’s operating point,
and hence, the lamp current, is set by
a potentiometer. Operating-point
variation can also be achieved by
voltage controlling the optional input, indicated on the schematic.1 With
the components shown, size is about
10mm (W) by 5mm (H) by 40mm (L).
The Shutdown pin facilitates circuit
turnoff, although removing power
from the VIN pin has similar results.
The closed loop operation yields
excellent line regulation while ensuring that lamp currents never violate
minimum or maximum values. These
characteristics allow operation
directly from the battery without
intensity variation, flicker or shortening of lamp life. Simplicity, low
component count, small size and cost
effectiveness make this circuit an
excellent choice for “palmtop” LCD
illumination.
1
Those finding this description intolerably brief
are directed to LTC Application Note 65, where
this circuit receives more scholarly attention.
BAV-99†
LAMP
DESIGN IDEAS
Cost and Space Efficient
Backlighting for Small LCD Panels
................................................... 27
6
3
10
T1
1
Jim Williams
5 4
3
2
2
1
750Ω
Q1
High Efficiency PolyPhase Converter
Combines Power from Multiple Inputs
................................................... 28
Wei Chen and Craig Varga
Isolated RS485 Transceiver
Breaks Ground Loops .................. 29
MBR0520
10µF
Mitchell Lee
Sharp Gain Roll-Offs Using the
LTC1562 Quad Operational Filter IC
(Part 3) ........................................ 31
Nello Sevastopoulos
Joseph G. Petrofsky
Q2
T1: TOKO BLC210
(408) 432-8281
L1: SUMIDA CD43-100
(847) 956-0666
Q1, Q2: ZETEX ZDT-1048 (DUAL)
(516) 543-7100
* WIMA MKI
(914) 347-2474
** 1% FILM RESISTOR
† MOTOROLA
(800) 441-2447
DO NOT SUBSTITUTE
COMPONENTS
L1
100µH
+
V+
(3V–9V TYP)
SHUTDOWN
Using the LT1719 Comparator for
Low Dispersion Sine Wave to
Square Wave Conversion ............. 34
0.15µF*
VSW
VIN
LT1317B
10k
SHDN
VC
+
1µF
FB
GND
22k
0.1µF
MMB0914
820Ω**
2k
DIMMING
0V–3V
(OPTIONAL)
Figure 1. Palmtop computer LCD backlight supply
For more information on parts featured in this issue, see
http://www.linear-tech.com/go/ltmag
Linear Technology Magazine • November 1999
27