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Wed Jun 10 17:04:22 CEST 2009

Charge pump

A charge pump[1] is a series of diode/capacitor circuits constructed
like this:

VDD --[>]--+--[>]--+--[>]--+--[>]--+--[>]--+--[>]--+
           |       |       |       |       |       |
          === C0  === C1  === C2  === C3  === C4  === C5
           |       |       |       |       |       |
           1       2       1       2       1       2

It operates by switching the 1 and 2 nodes at opposite voltages.
I.e. with 1=VDD and 2=GND, the 1 nodes supply a current that
discharges the attached capacitor, goes through the diode and charges
the next capactitor.  This works as long as VC_i+VDD is larger than
Vdiode+VC_i+1.

Because this circuit is current controlled, connecting a voltage
source to the nodes 1 and 2 will create a spike current.

What I wonder is why one typically uses inductor-based boost
convertors[4] for power supplies and not charge pumps.  Maybe diode
losses?  I had this crazy idea of using the bounces of a switch to
perform the initial transitions necessary to put a charge on a
capacitor enough to bootstrap a microcontroller into driving the pump
itself.

Apparently the term "charge pump" is used also in PLL[3] circuits,
which are used in voltage->frequency converters like [2].  This is
where I encountered it today.

[1] http://en.wikipedia.org/wiki/Charge_pump
[2] http://www.national.com/mpf/LM/LM2917.html
[3] http://en.wikipedia.org/wiki/Phase-locked_loop
[4] http://en.wikipedia.org/wiki/Boost_converter



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