TPS51206: VTT behavior at power up

Atsushi Takahashi

Part Number: TPS51206

Hi Team,

In the customer system, the attached waveforms were observed at power up.
Please see it.

VTT turns on with 2-steps, it rises to around 580mV first, then it rises to around 628mV.
Is this a problem?
If yes, please tell me a solution.

The data sheet, 8.2.1.2.5 VTTSNS Connection section, says as bellow.
"Consider adding a low-pass R-C filter at the VTTSNS pin in case the ESR of the VTT output capacitor(s) is larger than 2 mΩ.The R-C filter time constant should be approximately the same or slightly lower than the time constant of the VTT output capacitance and ESR."

For example, if they do this, is it improved?
However, they use ceramic capacitor as output capacitor.

Best Regards,

TPS51206_VTT_Power up.pdf

over 1 year ago

0 James Steenbock over 1 year ago

TI__Mastermind 19260 points

Hi Atsushi,

This E2E thread seems to show a similar behavior where VTT ramps up to a lower voltage before stepping up to the final regulation point: https://e2e.ti.com/support/power-management-group/power-management---internal/f/power-management---internal-forum/1260934/tps51206-tps51206-has-voltage-drop-problem-when-vtt-power-on/4774945?tisearch=e2e-sitesearch&keymatch=TPS51206%252520timing#4774945

This is likely related to the timing of your power up sequence. VTT does not reach the final voltage target until VTTREF is fully powered up. If you can ramp up VTTREF before VTT, this should remove the stepped behavior.

For best results I would recommend following the timings shown in the datasheet Section 7.3.6.

If changing the power sequence is not possible, you can still reach a stable operation state after VTTREF is powered up. The stepped behavior is not ideal but I don't think it will cause other problems as long at the downstream devices can wait for the correct VTT voltage. This is something that depends on your system.

Regards,

James

0 Atsushi Takahashi over 1 year ago in reply to James Steenbock

Expert 1130 points

Hi James,

Thank you for your answer.

I couldn’t access URL you sent.

Could you tell me correct URL?

0 James Steenbock over 1 year ago in reply to Atsushi Takahashi

TI__Mastermind 19260 points

Hi Atsushi,

I don't have an alternate link but I can share the image from the example thread below. I just wanted to use this as an example of what you might see if rails are ramped at the same time. (VTT ramped at the same time as VDD or VTTREF). Here you can see the same VTT stepped behavior on CH3 (CH1 = S3, CH2 = VDD).

Regards,

James

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TPS51206: VTT behavior at power up