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TPS51200: Maximum REFOUT capacitance

Shinichi Inoue
Guru 20301 points
Part Number: TPS51200

Dear Specialists,

My customer is encountering the problem when power is up.

I would be grateful if you could advise.

---

About DDR3 termination regulator
The recommended capacitance of the REFOUT output pin is 0.1uF.
Is it possible to change to 10uF?

About the regulator output voltage
The regulator itself has soft-start function, but overshoot has occurred.

I think that the power sequence used for the CPU may not match the recommended sequence for the regulator.
(3.3V = VIN rises after 1.5V = VLDOIN)

But, the CPU sequence cannot be changed.

When a capacitor of about 10uF was connected to REFOUT, overshoot was improved.
For this reason, I would like to confirm that there is no problem.

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Mastermind 24075 points

    Hi Shinichi, 

    Regarding the overshoot, do you refer to overshoot on REFOUT, overshoot on VTT voltage, or both?

    Do you have test waveform to share? 

    Also, can you try 4.7uF for REFOUT if the overshoot is acceptable?

    Thanks

    Qian

  • 0 in reply to Qian Chen
    Guru 20301 points

    Hi Qian,

    Thank you for your reply.

    I confirmed the customer.

    The overshoot appeared both REFOUT and VTT.

    I could get the waveform, could you please see attached file.

    REFOUT capacitance is 0.1uF, 2.2uF, 4.7uF and 10uF.

    With 0.1uF, it is  NG because overshoot  is over the CPU absolute maximum rating (1.1V).

    4.7uF will be about + 10% overshoot, so it seems to be acceptable, but I want a little more room.

    TPS51200_output_overshoot.xlsx

    I appreciate your great help.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Mastermind 24075 points

    HI Shinichi,

    Is it possible to pull EN high after 3.3V is stable?

    If it is doable, you can check if there is overshoot on VTT. 

    Thanks

    Qian

  • 0 in reply to Qian Chen
    Guru 20301 points

    Hi Qian,

    Thank you for your reply.

    I confirm the customer.

    VTT overshoot is the same as VREFOUT

    Could you please expand to 200%, you could see the waveform.

    Also the customer'd like to fix the overshoot by changing 4.7uF or 10uF.

    Could you please advise is it possible or not.

     

    Other waveform are the same, could you please expand. 

    I appreciate your great help.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 20301 points

    Hi Qian,

    This is just a reminder I am looking forward to waiting your reply.

    I really appreciate your great help.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Mastermind 24075 points

    Hi Shinichi

    Sorry for the late reply. 

    I remember other customer use 4.7uF before. So 4.7uF should be fine. 

    If customer want to use 10uF, can they add a several ohm resistor in series with REFOUT cap? The resistor helps to make REFOUT voltage stable when REFOUT cap value is very large. 

    Also, Is REFOUT cap close to TPS51200 REFOUT pin in layout? If not, can you place the cap as close to the pin as possible and check if overshoot become smaller?

    Thanks

    Qian

  • 0 in reply to Qian Chen
    Guru 20301 points

    Hi Qian,

    Thank you for your reply.

    I'll share your suggestion with the customer.

    If the customer has an additional question, I consult you again.

    I appreciate your great help in advance.

    Best regards,

    Shinichi