Control a processor`s power supply in real time

The ADSP-BF531 imposes several design requirements: Its core power-supply voltage must maintain its accuracy to within 25 mV and offer an adjustment resolution of 50 mV per step from 0.8 to 1.2V. Also, the processor requires 1.2V at start-up to initialize its clocks. Finally, the power controller must prevent its output voltage from exceeding 1.2V if a software glitch occurs. A digital potentiometer typically presents a highly variable absolute resistance value but can accurately set its internal resistance ratio. In this design, the AD5258's internal resistor forms a voltage divider with an external resistor to set the output voltage. To improve the ADP3051's output-voltage accuracy, the ADSP-BF531 uses a simple algorithm to compute and store an appropriate maximum resistance for a given operating voltage in the AD5258's nonvolatile memory via its I2C port. Using the AD5258 with an external resistor provides hardware protection to prevent the output voltage from going above 1.2V. If the AD5258 is set to zero resistance, the resulting output voltage is 0.8V×(0Ω+10 kΩ)/10 kΩ=0.8V.