LDO Improves White LED Ballasting

摘要

Since their invention a few years ago, white light-emitting-diodes (LEDs) have steadily gaining in popularity. Because they offer whiter light than incan-descent bulbs and are easier to use than fluorescent tubes, they have been steadily gaining market share in a variety of lighting applications such as flashlights and display backlighting in handheld electronics. However, white LEDs present their own set of technical issues. The most obvious issue when using white LEDs is that they are somewhat poorly matched. As an example, a typical white LED specification for forward voltage, Vf at 20 mA, may read 3.0 V minimum, 3.5 V typical and 4.0 V maximum. Obviously, a constant voltage source is not the solution. Although the recommended means of achieving matched brightness is to drive each LED with the same amount of current, this can be expensive. Therefore, most applications simply use a fixed bias voltage and ballast resistors, as seen in Figure 1.