/> With the trend towards higher integration, more peripherals are being packed into each microcontroller. However, when the key peripherals have been mapped, many designers find that other peripherals are simply left unused. With a little creative thinking and a handful of low-cost discrete components, the right mix of spare peripherals can be used to boost a 5V supply voltage to a low-current, regulated 15V.

The most obvious solution to boosting the supply would be to go to the expense of using an inductive boost. Another option would be to use a capacitive voltage doubler on the raw supply, but this would add another IC onto the board and also make the voltage susceptible to sagging under load.

A more creative approach is to harness unused peripherals on the microcontroller to create a simple capacitor-based voltage multiplier. Using the H-bridge-output modes of the microcontroller’s PWM, together with its voltage comparator, bandgap reference, and a few analogue-to-digital converter (ADC) channels, plus a few external capacitors, resistors and diodes, it is possible to create an enhanced version of a voltage doubler or charge pump. The original versions of these were used in the early days of television when the Villard cascade voltage multiplier was used to generate the high voltage for the CRT.

The microcontroller’s PWM provides a perfect charge pump, whilst the comparator and bandgap can be used as a simple feedback mechanism. Combining the charge-pump-multiplier topology with the PWM produces a circuit that could generate 18V unloaded. Regulating the output down to 15V is achieved by switching the PWM outputs on and off, based on voltage feedback.

With the trend towards higher integration, more peripherals are being packed into each microcontroller. However, when the key peripherals have been mapped, many designers find that other peripherals are simply left unused. With a little creative thinking and a handful of low-cost discrete components, the right mix of spare peripherals can be used to boost a 5V supply voltage to a low-current, regulated 15V.

The most obvious solution to boosting the supply would be to go to the expense of using an inductive boost. Another option would be to use a capacitive voltage doubler on the raw supply, but this would add another IC onto the board and also make the voltage susceptible to sagging under load.

A more creative approach is to harness unused peripherals on the microcontroller to create a simple capacitor-based voltage multiplier. Using the H-bridge-output modes of the microcontroller’s PWM, together with its voltage comparator, bandgap reference, and a few analogue-to-digital converter (ADC) channels, plus a few external capacitors, resistors and diodes, it is possible to create an enhanced version of a voltage doubler or charge pump. The original versions of these were used in the early days of television when the Villard cascade voltage multiplier was used to generate the high voltage for the CRT.

The microcontroller’s PWM provides a perfect charge pump, whilst the comparator and bandgap can be used as a simple feedback mechanism. Combining the charge-pump-multiplier topology with the PWM produces a circuit that could generate 18V unloaded. Regulating the output down to 15V is achieved by switching the PWM outputs on and off, based on voltage feedback.

Low-current, regulated, voltage boost circuit

Figure 1 shows that, when the output voltage is low, the first capacitor is charged for half a cycle through the comparator output. The charge is then transferred to the next capacitor on the other half of the cycle. After several more cycles, the charge propagates through the capacitors, adding about 4.5V per stage. When the output is sufficient to raise the inverting input of the comparator above the reference, the output goes low, and the first capacitor does not charge, preventing any new charge from propagating to the output.

A handful of resistors, capacitors and diodes and a few spare peripherals from a PIC® microcontroller can, therefore, be used to provide functions which would otherwise require the cost of an additional IC on the board.