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[Solved / Archived] Witty Pi Vin ripple

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Dobby
(@dobby)
Posts: 6
Active Member
Topic starter
 

Hello everyone,
We currently have a problem with our Witty Pis.
When booting with display connected, we noticed that the Witty Pi starts to speak very loudly. We have now measured the voltages and have discovered extreme ripples on the 12V Vin line. During booting, sometimes up to 4V at boot and ~1V at idle. To be on the safe side, we used different power supplies and batteries. All with the same result. See screenshot. After the Raspberry has booted and requires less current in idle, the ripples stabilize at approx 1V.
A quick check on the Witty shows only one capacitor each at the input and output of the MP4462 converter. Compared to the manufacturer's reference design EVQ4462-N-00A with up to 15 Capacitors, inductor and magnetic beads at Vin, does this seem a little low with these interference levels? Are there other users with such problems or already a workaround?

Thank you

1718786874-SCR64.png
 
Posted : 19/06/2024 10:47 am
(@admin)
Posts: 371
Member Admin
 

When evaluating a DC-DC step-down circuit, we measure the voltage ripple on its OUTPUT.

The huge 1~4V voltage variation you mentioned is on input side, and it indicates the input power source could hardly deliver enough current, and hence its voltage drops that much. A DC-DC circuit can not stablize the input voltage, it can stablize the output voltage only.

You may measure the voltage between the +5V and GND pins in GPIO header, and it should be within the 4.75~5.25V range.

 
Posted : 19/06/2024 11:03 am
Dobby
(@dobby)
Posts: 6
Active Member
Topic starter
 

I have now soldered another 100µ || 100n to both Vin and Vout. The WittyPi is now running stable again and no longer makes any noise. The voltage peaks were also on the Vout with up to 500mv. After the conversion it is less than 40mV. I can live with that.

 
Posted : 27/06/2024 11:47 am
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