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C6745 power rails - 3.3V

RandyP
Guru* 84110 points

In the C6745 datasheet SPRS377d section 6.3, it details the power-on sequence and the power-off sequence. In section 6.3.2 Power-off Sequence, it has the single line

SPRS377d 6.3.2 said:
The power supplies can be powered-off in any order as long as the 3.3V supplies do not remain powered with the other supplies unpowered.

Should we take this to mean only that the C6745 will not operate while only the 3.3V supply remains on (seems obvious), or rather that TI cannot guarantee the reliability of the device if it is powered with only the 3.3V supply being on and no others?

The statement under section 6.3.1 seems to also say strongly ("must") that 3.3V must be powered after the core voltage. Or does this mean it only matters for proper operation of the device rather than having any relationship to reliability?

SPRS377d 6.3.1 said:
group 4) must be powered-on after the core logic supplies

How should we interpret this for the case of a customer asking if they can power on the 3.3V rail for board tests? No time duration was specified.

Regards,
RandyP

  • 0
    TI__Expert 3540 points

    RandyP,

    Even though the IOs operate at 3.3V, all of the logic on the chip is built on 1.8V transistor technology. Powering the 3.3V supply without the other supplies puts voltage stress on the 1.8V transistors beyond the voltages they are designed for and could potentially cause long term reliability issues. Although short periods of time are probably not a problem (such as while the power supplies are turning off and bulk board capacitance is temporarily maintaining supply voltages), extended periods with the 3.3V supplies on and the other supplies off are not recommended. Unfortunately, it is very difficult to define how long is too long and the reliability impact has not been examined (since it is considered out of spec operation).

    Regards,

    Clay