dimming control for white-LED driver

Posted on Aug 5, 2012

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.

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