In battery-powered applications in which power management is key, a microprocessor may adjust its core voltage corresponding to an increase or a decrease in clock speed, allowing full processing power when necessary but not wasting excess power when idle. The circuit of Figure 1 shows how an embedded processor can control its own supply voltage via a simple step-down converter and inexpensive digital potentiometer.
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| Figure 1. | Under control of its host processor, digital potentiometer IC2 adjusts the processor’s core power-supply voltage. |
In this application, an embedded ADSP-BF531 Blackfin processor adjusts the wiper setting of IC2, an AD5258 digital potentiometer, via its I2C interface. In turn, IC2 controls the output of IC1, an ADP3051 current-mode, PWM step-down converter that supplies as much as 500 mA at output voltages as low as 0.8 V. When its output is in regulation, IC1’s feedback input rests at 0.8 V, and IC2 and R2 form a voltage divider.
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 1.2 V. Also, the processor requires 1.2 V at start-up to initialize its clocks. Finally, the power controller must prevent its output voltage from exceeding 1.2 V 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 1.2 V. If the AD5258 is set to zero resistance, the resulting output voltage is
If you set it to its maximum resistance of 5 kΩ, the resulting output voltage is
When the embedded processor directs the AD5258 via its I2C port to ramp the core voltage from 0.8 to 1.2 V, IC1’s output voltage monotonically increases within 40 µsec (Figure 2).
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| Figure 2. | Applying power-supply voltage to the host processor ramps voltage from 0.8 to 1.2 V with only 60-mV overshoot. |
