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Original question:

TMS320F28075: F28075 POR question

TMS320F28379S: Necessity of a reset IC

O.H
Mastermind 6660 points
Part Number: TMS320F28379S


Hi experts,

My customer is currently considering the F2837xS and we are discussing the need for a reset IC.

Taking into account design errors and margins, we believe that VDD is outside the recommended voltage range of 1.01 to 1.14V and may fall into the area where POR is not applied. Is this a problem if the time is short enough? If so, could you give me a quantitative upper limit?

I have the following understanding about the necessity of the reset IC. Refer to:e2e.ti.com/.../3538747

If VDDIO/VDD reaches 2.3V and 1.0V and falls within the Vmin of the device after 100us, the reset can be held properly. (The trigger point is about 2.3V for the VDDIO rail and about 1.0V for the 1.2V rail.) If not, it is recommended to use an external power supply monitoring IC (reset IC) or other device that holds XRSn until the power supply rail reaches Vmin.

For the above, I think you are assuming the rise of the power supply voltage at startup. In this time case, we are more concerned about the instantaneous voltage drop due to sudden load changes such as Flash Erase of the MCU after the power supply rises. When the maximum load described in the datasheet was applied in steps, a voltage drop of several 10mV was observed. I would appreciate your opinion on the necessity of a reset IC for this case.

Best regards,
O.H

  • 0
    TI__Guru* 81500 points

    O.H.

    You are correct, if there is concern about droop or even risk of external LDO failing during operation, then the on-chip XRSn gating from the power supply will likely not be sufficient in the customer system.  The risk is any operation of the device outside the Vmin limit before the reset logic can drive XRSn low per the above thresholds.

    In terms of droop I would recommend that the customer pick a LDO/power supply that can take this into account with its nominal voltage.

    Best,

    Matthew

  • 0 in reply to MatthewPate
    Mastermind 6660 points

    Hi Matthew,

    Thank you for your reply.

    Is there a time limit for operating outside the Vmin limit?

    I understood that the effect of instantaneous droop is within the absolute maximum rating, so there is no risk of failure, but the operation of the device is not guaranteed. We will recommend our customers to use power supply ICs that take this issue into account.

    Best regards,
    O.H

  • +1 in reply to O.H
    TI__Guru* 81500 points

    O.H.

    Unfortunately there is not a time of outside of spec operation we can give, if the device is operated out of spec we lose the ability to guarantee the performance will match the DS values.  So any time is at risk of the device not working correctly, and what that fault would be is unknown.

    Best,

    Matthew

  • 0 in reply to MatthewPate
    Mastermind 6660 points

    Hi Matthew,

    Sorry for late reply. I understood. Thank you for your support.

    Best regards,
    O.H