Cost and Space Efficient Backlighting for Small LCD Panels

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 turnoff, although removing power from the VIN pin has similar results.

Figure 1. Palmtop computer LCD backlight supply.

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.



Jim Williams

James M. Williams(1948年4月14日-2011年6月12日)是一名模拟电路设计人员兼技术文章作者,先后就职于麻省理工学院(1968–1979)、Philbrick、National Semiconductor (1979–1982)和凌力尔特公司(LTC) (1982–2011)。[1]他撰写了350多篇有关模拟电路设计的论文[2],包括5本书、21篇National Semiconductor应用笔记、62篇凌力尔特应用笔记以及超过125篇《EDN》杂志文章。 Williams于2011年6月10日中风,6月12日去世