Power supply ringing when servo activates


I'm running a servo from a 12V -> 5V buck regulator. The servo has a stall current of about 500mA and a nominal operating current of about 200mA. Here's a scope reading from when the servo starts moving:

(output side)

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(input side)

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The output voltage drops from 5.3V to 3.8V and then climbs back up, while the input voltage drops from 11.8 to about 10.2. The time scale is 20ms / div. I have a 250uA electrolytic cap near the servo power pin. Is this "ladder" normal? Or is something not right with my regulator?

The reg is an LM2734, which can do 1A, with a 4.7uH inductor with a 1.2A saturation point. The input is 8 AA Alkaline batteries.

Here's the servo circuit:

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And here's the regulator:

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Best Answer

Look at the time period of the peaks on the recovery of the bulk capacitance. By eye it looks like 5ms peek-to-peek, or ~200hz from your time base. The regulator datasheet reports the switching frequency of the buck-boost is internally set to 550kHz (LM2734Y) or 1.6MHz (LM2734X). This suggests the recovery waveform is dominated by something other than the boost injections of charge.

I might be tempted to try a diode between the supply side and the motor/bulk capacitance, on the assumption that there's some oscillation between the bulk capacitance and the charging L-C network that interferes with the feedback sense. You'd have to raise the output voltage of the buck-boost to compensate for the voltage drop across the charging diode

Is the converter a pre-assembled board or did you choose the various capacitor types yourself - what are they?