[eagle] CAN-Do Power Supply Instability

Juan Rivera juan-rivera at sbcglobal.net
Mon Jun 18 18:43:42 PDT 2007


All,

 

I dug into the CAN-Do circuitry today.  The CAN-Do module uses a Maxim
max1836 switching down-converter.  I found the following quote in the spec
sheet, "Commonly, instability is caused by excessive noise on the feedback
signal or ground due to poor layout or improper component selection.  When
seen, instability typically manifests itself as 'motorboating,' which is
characterized by grouped switching pulses with large gaps and excessive
low-frequency output ripple during no-load or light-load conditions."

 

It reminded me of the waveform I captured earlier from CAN-Do inductor, L3.
(See the waterfall display below.)  Those are definitely grouped pulses with
large gaps.  Notice also that there is tremendous splatter over a large
bandwidth that can just be seen as faint horizontal lines.

 

I'd like to propose an alternative - Move the switching down-converters out
to the power distribution and management area and feed DC to the payloads at
an appropriate voltage so switching power supplies can be removed from all
analog payloads.  This would go a long way towards cleaning up what is
shaping up to be a nasty situation.  It's just not good design practice to
bundle switching supplies with low-level analog signals.

 

73,

 

Juan



 

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