SN74AVCH2T45: Default output when input is floating after power-up

Shenhua Zhang

Prodigy 640 points

Part Number: SN74AVCH2T45
Other Parts Discussed in Thread: SN74AXC2T45

Hi Expert,

As there is internal bus hold circuitry, so TI don’t recommend using external pull-up and pull-down resistors.

Customer wants to know what’s the B1, B2 output (high or low) when A1,A2 are floating after first power-on?

Do they need to put R406 & R407 below to ensure the output state?

over 5 years ago

0 Dylan Hubbard over 5 years ago

TI__Mastermind 45725 points

Hi Shenhua,

Yes the initial output state will be unknown, but for default states you can add pull-up/pull-downs. They need to be quite large though, I'd recommend 100 kohm to not negatively impact the bus-holds. However, at this point there is no need to use the bus-hold device and I would recommend SN74AXC2T45.

0 Shenhua Zhang over 5 years ago in reply to Dylan Hubbard

TI__Prodigy 640 points

Hi Dylan,

Thanks for answering.

However, how to explain below statement in application note and datasheet?

In this AN section 6.3, it defines that the input will be low after power-on:

http://www.ti.com/lit/an/scla015b/scla015b.pdf In datasheet below, it is saying no pull-up/pull-down needed.

0 Shenhua Zhang over 5 years ago in reply to Dylan Hubbard

TI__Prodigy 640 points

Customer is now using 4.7k only, is there any problem?

0 Dylan Hubbard over 5 years ago in reply to Shenhua Zhang

TI__Mastermind 45725 points

Hi Shenhua,

Having a tendency to start up in a Low state is a little different than being a guaranteed Low output state after power up refer to this FAQ:https://e2e.ti.com/support/logic/f/151/t/737715?tisearch=e2e-sitesearch&keymatch=faq%3Atrue.

Bus-hold circuits typically have an output resistance between 1kohm and 5kohm. Adding a 4.7kohm resistor to a bus-hold input can create a voltage divider between the bus-hold output and the pull-up resistor, resulting in excessive current. For details on why there is excess current, please see this FAQ: https://e2e.ti.com/support/logic/f/151/t/737694

0 Shenhua Zhang over 5 years ago in reply to Dylan Hubbard

TI__Prodigy 640 points

Thanks, Dylan

Just a quick confirmation that customer wants, does it mean using 4.7k only is okay for function, but more power computation?

0 Dylan Hubbard over 5 years ago in reply to Shenhua Zhang

TI__Mastermind 45725 points

Hey Shenhua,

No, this will likely not be okay.

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SN74AVCH2T45: Default output when input is floating after power-up