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  • Logarithmic LED Dimmer

  • Created: Oct 13, 2014

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Summary

This circuit is an LED dimmer that uses DS1808 and MAX1573 components which drive the four LEDs to light logarithmically. With the use of DS1808, a dual audio taper potentiometer with logarithmic resistive characteristics and MAX1573, a fractional charge pump with regulated constant current for uniform intensity, the device can effectively adjust the total LED current from 1mA to 106mA, in 64 steps of 1dB each. It is designed for portable-power applications that require white LEDs with adjustable, logarithmic dimming levels.


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. To control the LED brightness, operational amplifier U2 monitors the difference between the high-side voltage and the wiper voltage of digital potentiometer U1. Moreover, it multiplies that voltage by a gain to set the maximum output current. Zero resistance at the potentiometer’s W1 terminal corresponds to minimum LED current, resulting to 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. Furthermore, 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.


Logarithmic LED Dimmer can be an effective lighting device in establishments like industries, offices or at home. This device can be used as ceiling lights, table lamps and bedroom lights. In addition, it can act as emergency light where the brightness level can be adjusted to conserve energy. However, as much as it can give enough light as desired, this can also alter the ambience of a certain environment that is convenient to the users.

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