Circuit delivers dimming control for white-LED driver

The demand for power in color cell-phone handsets is constantly increasing. New applications steadily emerge, making it imperative to reduce power consumption in the design. Examination of usage data shows that more than 50% of the power consumed is in providing backlighting for the color screen. People use the phones for playing games and MP-3s and a multitude of other heavy-duty, multimedia functions. Figure 1 shows the classic way to use a backlight-driver IC to provide dimming. By modulating an external PWM signal, the circuit controls the white-LED current. By adjusting the on/off-time ratio, or duty cycle, the circuit can provide drive ranging anywhere from full-on to full-off. This circuit relies on the fact that the baseband or application processor has a PWM timer available. A second method of obtaining dimming is to use an adjustable analog-input interface (Figure 2). The drawback of this approach is that it requires a DAC block in the digital-baseband or application processor.

This Design Idea presents a more widely adaptable approach to the dimming challenge. Although the LM4811 headphone amplifier is not designed to operate as a DAC, you can tweak it to do so. Figure 3 shows the dimming application. The implementation is straightforward. The output of the LM4811 attaches to the BRGT pin of the LM2794. The output current from the LM4811 is directly proportional to the digital value stored in the digital-volume-control block. The rising edge of the clock, along with the polarity of the Up/Down pin, sets the appropriate output current of the LM4811 and, thus, the output current of the white-LED driver. The resultant approach requires only two general-purpose input/output lines, which are available in all modern baseband and application processors. Moreover, this method requires zero processor cycles once the LED current is set. The approach is therefore optimal for both software and hardware.

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