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TPS54231: Output ripple during start up

Howard Zou
Genius 12365 points
Part Number: TPS54231

Hi,

the circuit is as below: we are converting 12V to 5V, we cut all the load on 5V power rail so it's 0 load.

When we float EN pin, we got the waveform below:

yellow-12V input, blue-EN pin, green- 5V output.

by zoom in we get the waveform:

by solder R406=200k, R430=40k, we can get normal start up waveform as below:

yellow-12V input, blue- EN pin, green- 5V output.

Why would floating EN pin cause the ripple of Vout, it looks like it stops and then restart, but floating EN pin should be default enable this device.

  • 0
    TI__Expert 4795 points

    Hi Howard,

    Could you help capture the SS voltage as well?  One possible reason is EN is sensitive to noise when floating.

  • 0 in reply to Vincent Zhang(BSR)
    TI__Mastermind 30550 points

    Hi Howard,

    The EN pin has an internal pullup current-source that enable the pin by default. I switching on the EN pin does not look right to me. can you please why EN is noisy like that. 

    The other thing I can notice from the plot is the VOUT is overshooting, then stops regulating, then VOUT drops, and the process repeats itself 3 times before it regulates to normal, this may be an indication of Inductor sizing or noise into FB pin. Can you please zoom in time to see the 3 spikes and include the SW, EN, SS, VOUT to see if the switching stops after the overshoot happens.  

    Thanks!

    Tahar

    thanks 

    Tahar