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TUSB4020BI: VDD, VDD33 Power up sequence

Ryo Akashi
Genius 9470 points

Part Number: TUSB4020BI

Hello experts,

I have question for power up sequence.
From datasheet P9, I thought the sequence is VDD (1.1V) -> (>10us) -> VDD33 when GRSTz is connected to capacitor to GND.
1. Could you tell the difinication of "be stable" in datasheet P9?
From EVM schematic, the sequence looks VDD33 -> VDD(1.1V). GRSTz is also connected to capacitor to GND.
2. Could you tell me which is correct power up sequence?
Thanks and best regards,
Ryo Akashi
  • 0
    TI__Guru 60870 points

    Hello Ryo,

    The condition that must be avoided is if 3.3V is ramped before 1.1V it is possible that the internal pullup resistor on GRSTz is enabled before the device is fully powered.  To avoid this condition, the customer must use an active reset circuit to hold GRSTz low or if they use a passive reset circuit 1.1V must ramp before 3.3V or simultaneously (like the EVM).

    Regards,

    JMMN

  • 0 in reply to JMMN
    TI__Genius 9470 points

    Hello JMMN,

    Thank you for your reply.

    I understand we should avoid 3.3V is ramped before 1.1V because it is possible that the internal pullup resistor on GRSTz is enabled before the device is fully powered. 

    You said EVM schematic can ramp 1.1V and 3.3V simultaneously. It means 1.1V can be ramped fully before 3.3V is ramped fully.

    Is it correct?

    Also, I would appreciate if you would tell me the difinication of "be stable" in datasheet P9.

    Thanks and best regards,

    Ryo Akashi

  • 0 in reply to Ryo Akashi
    TI__Guru 60870 points

    Hi Ryo,

    The EVM design allows 3.3V / 1.1V ramp to happen very close together, so that if the pullup on 3.3V is enabled early the GRSTz timing is still met.  Since customer designs have varying timing differences between the 3.3V ramp and 1.1V ramp, as well as differing ramp times the datasheet requirement (note 2 in section 7.7) about power on was added to prevent any possible issues.

    "Stable" on page 9 means the voltage ramp is completed.

    Regards,

    JMMN