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TPS51200: Total capacitance on REFOUT pin

Masashi Mineda
Expert 1155 points
Part Number: TPS51200

Tool/software:

Dear Sirs and Madams,

We use the TPS51200 as a power supply for DDR.

In designs that comply with the restriction "keep total capacitance on REFOUT pin below 0.47 μF" in the datasheet, cases have been confirmed in which overshoots exceeding the DDR3/4 specifications occur.

This issue has been reported in many E2E design support articles:

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/952470/tps51200-questions-about-overshoot

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1030554/tps51200-overshoot-observed-in-vo-and-refout

There are several other similar reports as well.

All of the responses from TI engineers, including the one above, suggest using a capacitance much larger than 0.47μF (100μF in some cases) to suppress overshoot, which contradicts the description in the datasheet.

Regarding this matter, I would appreciate it if you could tell me the following points:

1. Could you please tell us the background and advantages of adding "keep total capacitance on REFOUT pin below 0.47 μF" to the datasheet?

Why was the above sentence added to the data sheet in 2016?

2. A design using a large-capacity (e.g. 100μF) capacitor, as recommended by a TI engineer in E2E, does not meet the datasheet specifications, but is this an official solution?

3. In a design that complies with the capacitor capacity of 0.47uF specified in the data sheet and has only a DDR load (overshoot can be suppressed by adding a resistive load, but wasteful power consumption occurs), overshoot at the REFOUT output is almost inevitable, as in the case of E2E.

Regarding this point, if TI has any opinions or materials regarding the impact on DDR (e.g., the impact is not a problem), could you please provide it to us?

Regards,

Masashi

  • +1
    TI__Mastermind 22060 points

    Hi Masashi,

    1) From what I understand, the lower REFOUT capacitance recommendation is to keep the rise time shorter but there's no other function critical reason. If the rise time of REFOUT needs to be quick due to other dependent ICs then a larger capacitor might create complications.

    2) In my experience, overshoot can also be caused by not following the power up sequence correctly so this is the first solution I would check. Otherwise, yes, you can raise the REFOUT capacitance. We don't have an official workaround document but based on customer feedback, increasing the REFOUT capacitance seems to reduce the overshoot.

    3) We don't have any material on the overshoot impact. This would have to come from the DDR manufacturer.

    This device had been used without issue for many DDR products.

    Regards,

    James

  • 0 in reply to James Steenbock
    Expert 1155 points

    Hello James,

    Thank you for your advice.

    We selected 0.47uF for REFOUT according to the datasheet, but this did not reduce the overshoot.

    We also checked the sequence, but it doesn't seem like there is a problem.

    Therefore, we confirmed that overshoot can be reduced by changing REFOUT to 4.7uF.

    Regards,

    Masashi

  • 0 in reply to Masashi Mineda
    TI__Mastermind 22060 points

    Hi Masashi,

    Changing REFOUT to 4.7uF should be fine.

    Regards,

    James