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Power-down sequencing for UZEV

buckd

Hi,

 

We are designing a power supply for the UZ-EV based on the 'Power Sequencing' section of the designer's guide.  This guide says the reverse sequence must maintain on turn off and we are doing that. 

 

However, our voltage roll-offs during the turn-off sequence takee significantly longer than that of the carrier card - our common case takes about 500-1000 ms, whereas the CC common case takes about 100-150 ms.

 

Will this be a problem during turn off?


We could also put 5.1K resistor in parallel to ground on the output of our linear regulators to match the roll-off time, at the cost of a few hundred microamps.

 

Thanks

  • On the Avnet designed boards we do not issue a controlled shut down as to do so would require something to "stay alive" after power off. In our system power down triggers a shut down by all rails simultaneously (plug gets pulled). The recommended sequence ensures minimum current draw, which during shutdown isn't really a big deal in most applications. In 7 series devices there was a potential issue if a VCCO bank exceeded VCCAUX by more than 2.65V for longer than 500ms. That limitation isn't documented any more so in the newer devices I don't believe it is an issue. If you have no VCCO banks higher than 2.65V then you won't ever have that condition. If you do, then you still most likely will not have an issue as the 2.65V delta is in reference to the 1.8V supply. That means that 1.8V supply would have to go to 0 very quickly for any Vcco bank to exceed it by 2.65V. Let me know if that makes sense, if not I can try and provide more details.

    Thanks
    Chris

  • in reply to ctammann

    Yes, that makes sense.  Thank you very much for the info.