64-tap LED linear brightness controller uses a digital potentiometer to control LED brightness

Applications that use LEDs but lack a microcontroller or other form of control can benefit from a simple circuit that allows manual control of the intensity of the LEDs. Devices suitable for this purpose include mechanical (analog) and electronic (digital) potentiometers. Digital potentiometers are smaller, more reliable, and generally less expensive alternatives to mechanical potentiometers, with up and down buttons (Figure 1).


IC2 is a current regulator that drives a string of LEDs at up to 200 mA. In a standard application circuit, IC2's internal regulator senses the voltage drop across a current-sense resistor, RSENSE, connected in series with the LED string. Thus, IC2 controls the current through the LED string by adjusting the voltage on the differential inputs CS- and CS+ to produce a setpoint of 204 mV on RSENSE. Resistors RA and RB cause the output voltage at pin 6 of IC1 to adjust the current level. IC1 is a 64-tap linear digital potentiometer with a resistor connection between ground and V5, a well-regulated voltage generated internally by IC2. You can manually adjust the RW control voltage (pin 6), which is part of V5, using the up and down buttons. You can quickly and easily calculate the required resistor values ​​by making a few assumptions. To begin, you can fix RA and calculate RB and RSENSE. Assuming you can ignore the maximum 6.93mA error caused by the bias current at CS+, you will choose a value for RA that is much higher than the equivalent resistance of IC1, which has a worst-case resistance of 2.9kΩ (the sum of the upper and lower resistors and the series resistance of the actuator) at the 32 position. RSENSE is much lower than RB.

When RA is set to 25.5kΩ, VWIPER = (5V/63) × N, where N is the actuator setting (0 to 63). Then solve the equation: (VWIPER - 0.204V)/RA = (0.204V - ILED × RSENSE)/RB. Solving for RB from this equation, with ILED = 0, N = 63, and VWIPER = 5V (top position): RB = 25.5kΩ × 0.204V/(5V × 0.204V) = 1.085kΩ. RB can be selected as the standard 1.07kΩ (1% series) or 1.1kΩ (5% series). At the bottom position, VWIPER = 0, the LED current is the maximum 200mA, and the brightness should be the maximum available. Calculating RSENSE, RSENSE = [0.204V + (0.204V × (1.085/25.5))] / 0.2A = 1.063Ω; the standard value selected from the 1% series is 1.07Ω.

The plot of LED current vs. wiper position shows a slight nonlinearity due to the change in resistance seen by the wiper at different wiper positions (Figure 2). At the two ends of the potentiometer, only the 400Ω wiper resistance is seen. As the wiper moves toward the midpoint, the resistance increases to one-quarter of the maximum end-to-end resistance. Since IC1 is a 10kΩ potentiometer, the wiper resistance seen at the midpoint is approximately 2.5kΩ in series with RWIPER. This variation results in a maximum linearity error of 8%, which is negligible in most LED applications. IC2 provides protection from overheating and overload conditions. For more efficient power dissipation and to avoid thermal cycling, the exposed tap of the package must be connected to a large ground plane.