The increasing use of color LCDs in handheld equipment has created a need for smaller and cheaper sources of white backlight. In the past, cold-cathode fluorescent lamps (CCFLs) and electroluminescent (EL) panels have been employed. But such circuits are excessively large, expensive, and complex for today's handheld consumer electronics devices. Fortunately, recent advances in LED technology have produced LEDs that emit white light. White LEDs have several advantages over conventional backlight types, including small size, low cost, minimal complexity, and high reliability.
To obtain white light, an LED is simply forward-biased (the typical forward-bias voltage for white LEDs is about 3.5 V ±10%). A boost circuit is generally required since the white LED's forward voltage is often close to or greater than the battery voltage.
The conventional approach to this problem relies on a boost regulator that biases the LEDs via a ballast resistor. This arrangement has its drawbacks, however. For instance, the wide variation of forward voltage in white LEDs causes a large variation in bias current and the resulting light output. Also, the conventional boost converter has a dc path between the input and the output (even in shutdown) that allows an inactive LED to drain the battery.
This compact circuit overcomes these problems (see the figure). U1 is a regulated buck/boost charge pump in a small µMAX package, with 100-mA output-current capability. Configured as shown, the circuit directly regulates bias current flowing through the white LED. By biasing multiple white LEDs in parallel, good light distribution can be achieved. U1's design eliminates the troublesome input-output path in shutdown. Its —SHDN input (pin 2) lets users turn the backlight on and off. The circuit also includes a power-OK output (POK), which signals a microprocessor when the backlight is available.
Though not necessary in this case, the input RC π - filter limits the voltage ripple reflected back to the input to just 40 mV p-p (for VIN = 3.6 V). Since the output voltage ripple is not visible to the human eye, it's of secondary concern in this application. This allows for the use of a small (0.22 µF) output capacitor. Even with this small output capacitor, the output ripple is only 40 mV p-p.