Keywords: portable power, white LEDs, logarithmic dimming, charge pump, digital potentiometer
White-LED Driver Provides 64-Step Logarithmic Dimming
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:
R5 = 215x0.6/ILED(Desired)
where ILED is the current through one LED.
U1 is a digital potentiometer with logarithmic taper and an analog-voltage wiper for which each tap corresponds to 1dB of attenuation between H1 and W1 (pins 11 and 9). It contains two pots controlled by a 16-bit code via a 3-wire serial interface. To set the LED current, drive RST-bar high and clock 16 bits into the D terminal of U1, starting with the LSB. Each pulse at CLK enters a bit into the register.
The circuit uses only one pot, so bits 0 through 7 are "don't care" bits. Bits 8 through 14 determine the wiper position: bits 8 through 13 set the code, and bit 14 is "mute." (Logic one at bit 14 produces the lowest-possible output current by setting the left side of R5 at approximately 0.599V.) After entering all 16 bits, enter the code and change the brightness level by driving RST-bar high. Figure 2 shows the logarithmic relationship between an LED current and the pot's input code.
Figure 2. LED current vs. input code for the Figure 1 circuit.
|DS1801||Dual Audio Taper Potentiometer||Samples|
|MAX1573||White LED 1x/1.5x Charge Pump in UCSP and Thin QFN||Samples|
|MAX4480||Single/Dual/Quad, Low-Cost, Single-Supply, Rail-to-Rail Op Amps with Shutdown||Samples|
|EE-Mail||Subscribe to EE-Mail and receive automatic notice of new documents in your areas of interest.|
|Download||Download, PDF Format (39kB)|
|© Sep 08, 2004, Maxim Integrated Products, Inc.|
APP 3287: Sep 08, 2004
APPLICATION NOTE 3287, AN3287, AN 3287, APP3287, Appnote3287, Appnote 3287