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
current 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 turn-off,
although removing power from the VIN
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 or 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. 
Figure 1. Schematic diagram of small, cost efficient LCD backlight supply
Vendors of non-LTC components used in this application
1 Those finding this description intolerably brief are directed to LTC Application Note 65, where this circuit receives more scholarly attention.