Diode ladder multiplies voltage under software control

The circuit in this Design Idea uses a Microchip 12F10 controller to drive a voltage multiplier ladder and single pin to output status and input a trigger signal you supply (Figure 1). When you trigger the signal, the software turns on a MOSFET to connect the multiplier output to a load. The microcontroller has an internal comparator with a 0.6V trip point. The circuit attenuates and feeds back the output voltage to this comparator.



Listing 1 shows the controller-based software, which stops the oscillator, driving the voltage multiplier when the internal comparator indicates that the output voltage has reached an upper limit. This circuit works in a wireless-monitor design, increasing the voltage, power, and range of a small periodic transmitter. It can provide 12V to 15V, and 9mA to 11mA.

Processing begins when power is applied. The controller qualifies its Port 3 input on Pin 4. When at a logic high, logic is true, and the software code generates complementary PWM outputs on ports 4 and 5, which are pins 3 and 4, respectively.

These oscillations charge the ladder network. The controller outputs a low on the Port 2/Pin 5 status line, indicating that charging is under way. You choose the ratio of R1 and R2 so that the center node of the ladder is at 0.6V when the output voltage reaches the desired value. When the output reaches the final value, the controller puts the status pin in tristate mode, and the 20kΩ resistor pulls the pin up to the power-rail voltage. Port 2 on Pin 5 then becomes an input.

When you pull this pin low, the microcontroller asserts Port 1 and Pin 6 high, turning on the P-channel MOSFET through Q2, and applies the output voltage on C4 to the load. Meanwhile, Port 1 and Pin 6 go high, shifting the lower pin of output capacitor C4 from ground to the power rail and adding a few volts to the output of the voltage ladder.

The program drives the complementary outputs at pins 2 and 3 with a 700μsec PWM period with a 50% duty cycle. You can change the software code to vary these parameters. The controller has an internal 4MHz oscillator and supports a user-settable reference block. The code continues to monitor the enable pin, C4’s voltage feedback, and the pump operation during the discharge to the load. You must set certain bits in the processor configuration for this code to work (Figure 2).