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DRA829J: Questions about power-down Sequencing

Tank.Xue
Prodigy 90 points
Part Number: DRA829J
Other Parts Discussed in Thread: TPS6594-Q1, , DRA829

Hi

We follow the PDN-0C (two TPS6594-Q1 devices) to power supply the DRA829J processor. The MCU and Main voltage domains are Isolated.

In our design, the power-up sequencing is correct, as shown below figure.

And it matches <Isolated MCU and Main Domains Power- Up Sequencing>, described in the section 7.10.2.4 of DRA829's datasheet.

However, there are some doubts in our power-down sequencing.

Shown below figure.

In our desing, we are shutting down the system by directly disconnecting the power source. But the power-down sequencing does not match <Isolated MCU and Main Domains Power- down Sequencing>, described in the section 7.10.2.5 of DRA829's datasheet.

Is this power-down sequencing allowed?

Whether this mismatched power-down sequencing may cause some unpredictable problems?

How should we modify our design?

Best Regards,
Tank

  • 0
    TI__Expert 7065 points

    The scenario your describing is an "Abrupt Power Down" where the main input source is abruptly disconnected without any warning. The PMIC device will begin a normal "Controlled Power Down" seq as the input VSYS_3V3 begins to discharge & PMIC_EN derived from 1st stage PG signal asserts low at ~ -5% of 3.3V output. PMIC will typically execute ~3ms of controlled power down steps as VSYS_3V3 discharges towards the PMIC input min ~2.7V. This is sufficient time to protect the SOC by executing first keys power down steps:
    1. Asserting PORz low to put SOC into safe reset state
    2 Disabling high current power resources (i.e. VDD_CORE, VDD_CPU, VDD_DDR) which are disabled within 2.5ms following PORz being asserted low.

    You can see from your scope shot the "3V3" rails are all reflecting the VSYS_3V3 voltage discharge slope and it looks like several 1.8V supplies are being disabled at ~3ms before PMIC loses control and remaining supplies discharge towards Gnd.

    Yes, an Abrupt/Uncontrolled Power Down seq is allowed. There are no known POH reliability issues related to this type of power down.