[2604] How Charge Pumps Work

Date: August 9th, 2019 | Comments : [2] | Categories: DIY.

Charge pumps are fairly simple devices. They involve charging a capacitor to the voltage of the supply, and then stacking it on top of that power supply so that the voltage potentials are added together, effectively doubling the output.

In the circuit above, there is a square wave coming from pin 3, and it alternates between 0v and 9v. The driver transistors of the 555 are pulling pim 3 up and down to the power supply limits. (We will ignore the voltage drops across D1 and D2 for now.)

When pin 3 is 0v, the capacitor C3 charges to the value of the voltage input (9v). So the voltage potential across the plus and minus connections of C3 are 9v.

When pin 3 is 9v, the capacitor C3 (that has the 9v potential stored on it) is referenced on its negative end to 9v, so the voltages add up. The 9v output from pin 3 and the 9v stored on C3 are added together to give the 18v output, which is sent to the output capacitor C4 to be stored for use. The C3 voltage is stacked on top of the output voltage from the 555 pin 3.

When pin 3 toggles back to 0v, C3 begins to charge back up and the process starts over.

The diodes D1 and D2 keep voltages from flowing in the wrong direction. There is a voltage drop from each diode, so C3 can never charge all the way to 9v, and some of the stored voltage is lost in D2 when it is charging the output capacitor C4. If you have a 0.5v loss in each diode, then the output will be 17v instead of 18v.

Charge pump ICs use mosfet switches to pull the end of the capacitor alternately to 9v and ground, but the general idea is the sameā€¦ charge up a capacitor and then stack it on top of the voltage supply in order to double the voltage.

There will almost always be some ripple in the pumped voltage output, but in many circuits it will not be noticeable. In others, like Klones, it can be a problem, especially if it leaks onto the power rail and goes into other pedals that do not have good filtering. The noise from poorly filtered charge pumps will most often be heard as a background whine in some pedals, especially flangers, chorus, analog delays and some digital fx.

For most charge pumps, its noise has the appearance of a rough sawtooth wave. This is caused by the fact that the output capacitor C4 is receiving a dump of charge only 50% of the time from the switched capacitor. The other 50% of the time it is having to provide all of the current to the circuit and its charge slowly bleeds off until the next current dump.

Pump up; bleed off. Pump up; bleed off. Pump up; bleed off.

Good, solid low impedance ground connections are essential. Proper filtering techniques are essential. Daisy chains should be avoided with pedals containing a charge pump.


2 Responses to “How Charge Pumps Work”

[786664] admin Says: 11:47 am, August 9th, 2019

In a previous blog post, I showed a dc-dc converter that is not a charge pump. It is more efficient and has the advantage of isolating the output from the input.


[786672] Howie Says: 9:50 am, August 12th, 2019

Great, clear explanation; thanks!


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