SN74AVCH2T45: What would possible happen if Vccb is before Vcca powered up

Hi Josh,

Thanks for the info, could you pls help explain more details about why powering up the Vccb first will cause the spike?

I saw in the datasheet that both Vcca and Vccb's maximum voltage rating is 4.6V, does that mean that as long as Vccb's spike is not above 4.6V, customers can power up the Vccb first? 

Thanks.

BR,

Manu

Hello Manu,

It has to do with powering up output buffers before inputs buffers. Meaning if DIR= High and VCCB is powered up first, there is a possibility a spike or glitch will appear on Bx inputs. Similar scenario if DIR=Low and VCCA is powered up first. 

The spike voltage will be equal to VCCB(DIR=H) or VCCA(DIR=L).

Regards,

Josh

Hi Josh,

So are you meaning that as long as the output stage supply is powered up after the input stage supply, this device is all good? Not specifically Vccb is must powered up after Vcca, is it also related with the direction?

BR,

Manu

Hello Manu,

That is correct.

Another way to prevent false logic levels from appearing is having one supply at GND during power-up. This will force all I/Os into a Hi-Z state

Regards,

Josh