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MC34063 gives me ADC noise on AVR! :(

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scrat:

--- Quote from: jahonen on March 07, 2011, 07:26:40 pm ---Can't it be described shortly without philosophizing by just saying that internal oscillator runs with 85% duty cycle if sensed voltage is below the reference and oscillator is completely off when sensed voltage is above the reference? I think it can be seen from the attached scope screenshot (measured from the switch node).

It also explains why the ripple is so peculiar, there are times where switching frequency drops very low (long time between bursts).

Regards,
Janne

--- End quote ---

Well, it can have duty-cycle between zero and the maximum, since it can switch on AFTER the sync from the oscillator.
That was just my view of the thing, a model that (for me) gives best insight into the behaviour. I think that a lower cap value (and an additional filter, as in one example in the app note) could avoid that oscillation, at least for a range of load values.

Alex:
Great info guys, thanks for your help exhausting this issue. I would have not brought this up if I had not seen a working (!) published design in a popular electronics mag. Hopefully this will help eliminate such designs.

Alex

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