The PWM (pulse-width-modulation) output available from many microprocessors is based on an internal 8- or 16-bit counter and features a programmable duty cycle. It is suitable for adjusting the output of an LCD driver (Figure 1), a negative-voltage LCD driver (Figure 2), or a current-controlled LED driver (Figure 3). The circuit comprises simply the PWM source, capacitor C, and resistors R_D and R_W. For CMOS circuits, you calculate the open-circuit output voltage as V_CONT=D×V_DD, where V_CONT is the control circuit's output voltage, D is the PWM duty cycle, and V_DD is the logic-supply voltage. The control circuit's output impedance is the sum of the resistor values R_D and R_W: R_CONT=R_D+R_W. For the circuit of Figure 1, the output voltage, V_OUT, is a function of the PWM average voltage, V_CONT:

where V_REF is the reference voltage at the feedback input.
Bear in mind that the initial charge on filter capacitor C produces a turn-on transient. The capacitor forms a time constant with R_CONT, which causes the output to initialize at a voltage higher than that intended. You can minimize this overshoot by scaling the value of R_D as high as possible with respect to R₁ and R₂. As an alternative, the microprocessor can disable the LCD until the PWM voltage stabilizes. For Figure 2, the output voltage, V_OUT, is a function of the PWM average voltage, V_CONT:
 

where V_REF is the reference voltage at the feedback input. For Figure 3, the output current is a function of the PWM average voltage, V_CONT:

where V_REF is the reference voltage at the Set output and K is the current-scaling factor.

R_D isolates the capacitor from the feedback loop in the PWM-control methods. Assuming a stable voltage at the feedback point, the following equation defines the lowpass filter's cutoff frequency: f_C=1/(2πRC), where R=R_D||R_W. To minimize ripple voltage at the output, you should set the cutoff frequency at least two decades below the PWM frequency.

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