AN3287| Application Note
Maxim/Dallas > App Notes > AUTOMOTIVE
DIGITAL POTENTIOMETERS
DISPLAY DRIVERS
Keywords: portable power, white LEDs, logarithmic dimming, charge pump, digital potentiometer
Jul 20, 2004
APPLICATION NOTE 3287
White-LED Driver Provides 64-Step Logarithmic Dimming
This circuit drives as many as four white LEDs in parallel from a 3.3V source, and adjusts the total LED current from 1mA to 106mA, in 64 steps of 1dB each. The circuit of Figure 1 is designed for portable-power applications that require white LEDs with adjustable, logarithmic dimming levels. It drives as many as four white LEDs from a 3.3V source, and adjusts the total LED current from 1mA to 106mA in 64 steps of 1dB each. The driver is a charge pump that mirrors the current ISET (sourced from U3's SET terminal), to produce a current of (215ISET 3%) through each LED. Internal circuitry maintains the SET terminal at 0.6V.
Figure 1. This circuit provides a logarithmic-dimming capability for white LEDs. To control the LED brightness, op amp U2 monitors the difference between the high-side voltage and the wiper voltage of digital potentiometer U1. The op amp then multiplies that voltage by a gain to set the maximum output current. Zero resistance at the pot's W1 terminal corresponds to minimum LED current, and therefore minimum brightness. Because the SET voltage is fixed (at 0.6V), any voltage change at the left side of R5 changes ISET, and the resulting change in LED currents changes their brightness level. R5 sets the maximum LED current:
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