With the increasing use of color LCDs in handheld devices , there is a need for compact and inexpensive white backlights. Conventional solutions use cold cathode fluorescent lamps (CCFLs) and electroluminescent (EL) plates, but these circuits present large size, high cost, and high complexity for current handheld consumer products. The emergence of white LEDs has solved these problems well. White LEDs have many advantages over traditional backlights, including small size, low cost, low complexity, and high reliability.
The white LED's forward voltage is typically about 3.5V ±10%, and white light is only needed to provide a forward bias to the device. A booster circuit is required when the white LED's forward voltage is higher than the battery voltage. The traditional solution is to use a boost regulator to bias the LED through a ballast resistor. This solution has two drawbacks: first, the white LED is wider. The forward voltage variation range will cause a large bias current change, resulting in brightness deviation. Second, the traditional boost converter has a DC path between the input and output (even in the off state), making it inoperative. The LED unnecessarily consumes battery current.
The circuit below is a compact solution that overcomes the above drawbacks (Figure 1). The regulated step-up/step-down charge pump is available in a small μMAX package (U1) that delivers 100mA of output current. According to the configuration in the figure, the circuit can directly provide a stable bias current for white LEDs, and has a better brightness distribution when biasing multiple parallel LEDs. The U1 circuit has no DC path between the input and output in the off state, and the user can control the on and off of the backlight with its active low SHDN input (Pin 2). The circuit also has a power-good (POK) output that tells the microprocessor that the backlight is ready.
Figure 1: Breaking the conventional connection allows the regulated charge pump IC to directly adjust the bias current of the white LED.
Although not necessary in this Case, the input RC "π" type filter limits the voltage ripple reflected to the input to only 40mVP-P (VIN = 3.6V). Since the output voltage ripple does not affect the visual effect, it is considered a secondary position in this application, allowing a small output capacitor (0.22μF) to be selected. Even so, the output ripple is only 400mVP-P. (Edit: Technology)
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