Author Topic: Derating caps for charge pump  (Read 708 times)

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Offline Sigmoid

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Derating caps for charge pump
« on: February 22, 2015, 03:54:47 am »
I'm a bit uncertain about capacitor voltage ratings in a charge pump scenario. :) I don't want to waste space, but I know that ceramic capacitors need to be seriously derated as their capacitance droops as we come close to their rated voltage...

So in relation to the voltage output of the charge pump, how should the capacitors be rated? Does the switching frequency affect this choice?
 

Offline T3sl4co1l

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Re: Derating caps for charge pump
« Reply #1 on: February 22, 2015, 07:44:30 am »
Switching frequency affects value but not voltage.

Typically, if you'll be choosing X7R type caps (I wouldn't recommend anything worse e.g. X5R, Z5U), using no more than third or half of rated voltage is recommendable.

For a charge pump, you can use excessively large values for the operating frequency (assuming the chip is OK with this) and current flow will be limited by channel resistance of the switches, rather than reactance.  The only consequence of using nominal value (or less) capacitances is increased supply resistance.

Unless you're severely constrained on cost and space, I should think this isn't a problem; charge pumps are rarely used beyond +/-12V or so, and at low currents (< 10s mA?).  A regular 0805 at 50V and 0.1uF should be a good starting point, or up to 1uF in the same size I think.  You'd have to compromise on value, voltage or performance to use smaller or cheaper parts.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
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