Charge pump output resistance


I am designing (simulating in LTSpice and building on a breadboard) a charge pump to power my numitron tube. I need to get ~15-20 V out of 3.3 V. The circuit is as follows:


simulate this circuit – Schematic created using CircuitLab

All diodes are 1N4148, the V1 source comes from a voltage regulator, while CLK1 from an Atmega OSC1 pin (Atmega powered by the same voltage regulator). The NOT gate is made from a NAND gate on a separate IC.

The circuit gives me about 18-20 V on the output without load, but when loaded, it lowers drastically. I measured the effective output resistance of this charge pump to be about 11 kOhm which is way too much for me. I tried simulating the circuit with smaller/bigger capacitances or frequencies, but it didn't change much.

The question is: how can I lower the output impedance of my charge pump?

Best Answer

The basic output impedance should be like this

$$ R_{eq} = \frac{N}{f*C} $$

So, increase f or C.