Aaron Lager, Masterwork Electronics, Santa Rosa, CA
( 01 Aug 2006 )
The combination of an external circuit and a low voltage microcontroller occasionally requires a significantly higher power-supply voltage. You can use either an external boost converter to increase the logic supply or a buck converter to decrease an even higher voltage. However, you can alternatively use the microcontroller to create a higher voltage. For example, some of Cypress Semiconductor’s (www.cypress.com) PSOC (programmable-system-on-chip) microcontrollers include a configurable comparator block that, with a PWM block, can form the heart of a simple inductor-based boost converter (Figure 2). A few external components implement a 40V power supply (Figure 2). When the feedback voltage you apply to Pin 3 (P0.3) exceeds the comparator’s software-defined threshold voltage, the comparator shuts off the PWM stage. When the voltage drops below the threshold, the comparator re-enables the PWM block and thus regulates the output voltage. The voltage regulator uses only hardware blocks and thus is immune to the effects of other activities taking place in the PSOC’s CPU.
However, some microcontrollers lack a built-in comparator. For these devices, the Villard Cascade circuit offers a less expensive alternative to an external boost-voltage converter (Reference 1). Most engineers who are familiar with the Villard Cascade associate it with high voltage applications and do not envision it as a low-voltage dc supply technique. The circuit in Figure 3 requires an ac input source that you can easily simulate using a PSOC’s internal PWM and inverter blocks. A square-wave output voltage appears on Pin 1, and an inverted version of the same square wave appears on Pin 2. The voltage difference between the two pins applies an ac square-wave voltage to the cascade.
Figure 4 shows how to configure a PSOC’s internal blocks to drive the circuit in Figure 3. The PSOC’s output multiplexer inverts the PWM’s output and drives Port_0_5, and Port_0_6 receives the PWM’s noninverted output signal. Again, the PSOC uses hardware blocks to drive a Villard Cascade voltage multiplier, and the circuit produces an output voltage without regard to CPU activity. For an input voltage, VIN, a Villard Cascade of N stages delivers an output voltage of VIN 2N. One stage comprises two diodes and two capacitors (Figure 5). However, the series connected capacitors and diodes introduce voltage drops that limit the output current available from a Villard Cascade. In addition, the following equation imposes a practical limit that governs the cascade’s output voltage:
where ΔV is the output-voltage drop, f is the input frequency, C is the capacitance, I is the output current, and N is the number of stages.
Both boost circuits can supply only modest amounts of current, especially when they receive power from a 5 or 3.3V source. However, you can charge a high-value storage capacitor from the boost circuit’s output and drive a load that presents a low duty cycle (for example, solenoid actuation).
