Adjusting the output voltage of a power supply is a feature typically reserved for highly integrated handheld power-management units (PMUs) with multiple integrated power supplies. You can implement this feature, however, with any power supply by adding a single resistor and an inexpensive digital-to-analog converter (DAC) (Fig. 1). While this technique isn’t new, the equations to select appropriate resistor values are rarely published.
Using nodal analysis on the circuit in Figure 1 provides the fundamental equations:
You can easily derive the equations for the three resistors using these fundamental equations and the knowledge that the DAC voltage is inversely proportional to the output voltage. For example, when VOUT = VOL, then VDAC = VDACH, where VOL = the power supply's lowest output voltage and VDACH = full-scale DAC voltage.
Then, using Eq. 1 and Eq. 2:
So, when VOUT = VOH, then VDAC = VDACL, where VOH = highest output voltage and VDACL = zero-scale DAC voltage.
Again, using Eq. 1 and Eq. 2:
Substituting and solving for R1, R2, and R3 yields:
The circuit in Figure 2 uses the LM5116 power-supply controller, which has a threshold voltage of 1.215 V, to generate an arbitrary dynamic output voltage ranging between 0.6 V and 5.1 V. The output voltage is adjusted by the DAC081S101, which swings between 0 V and a reference of 5 V. The calculated resistor values for this example are R1 = 3.73 kO, R2 = 8.66 kO, and R3 = 9.1 kO.
Returning to the fundamental equations, you can calculate VO as a function of the DAC voltage, VDAC:
The format of Eq. 8 is y = mx + b, which proves that the output voltage is a linear function of the DAC voltage. Figure 3 plots the measured output voltage versus the DAC voltage for the example resistor values calculated above.
When using this method to control your output voltage, ensure that the DAC can drive the chosen resistor values. A unity-gain operational amplifier may be needed to buffer the DAC output. Alternatively, choosing resistor values in excess of 10 k allows just about any DAC to meet this requirement. If you plan to program the output voltage to a value below that of the controller threshold voltage, the DAC must be able to drive a voltage higher than the threshold voltage.