SN74AVC4T245: VCC isolation

Nicholas Carley

Part Number: SN74AVC4T245

Hello,

I'm running into an issue on a handful of boards where when one of the VCC rails goes low (VCC isolation event) the SN74AVC4T245's second power rail pulls ~100uA-400uA more current.

In this case, we bring VCCa to 0V (which is also tied to OE) and the VCCb side is still high. We see an additional ~100uA-400uA pulled from our battery and removing the SN74AVC4T245 stops the issue from occurring. We have 2 SN74AVC4T245's on our board but only 1 sees this issue and its not consistently the same device.

Its my understanding from the datasheet that if one power rails goes to 0V that the outputs should go high impedance and the device should pull no more than ~5uA, is that correct?

If not, how much current should we expect to see during this situation?

Thanks,
Nick

over 5 years ago

0 Karan Kotadia over 5 years ago

TI__Genius 16735 points

Hi Nick,

You are correct, the device does go into high impedance outputs when one of the supply is at Vcc=0 and this disables the ability to pass through information. However, this does not disable the CMOS inputs of the device. You still need to have valid logic states at the input of the device, either Vcc or GND. Having floating or inputs at 1/2Vcc can cause large amounts of current draw which you see in your case.

https://e2e.ti.com/support/logic/f/151/t/737694?tisearch=e2e-quicksearch&keymatch=FAQ%20floating

Thanks!

-Karan

0 Nicholas Carley over 5 years ago in reply to Karan Kotadia

TI__Genius 9915 points

Hi Karan,

Thank you for that FAQ. Just for clarification, when you say "floating inputs" which pins are we talking about?

OE/DIR pins?
1A/1B and 2A/2B pins?

So, If the VCCa side goes to 0V and there is still a voltage applied to the B side input pins, that is okay, so long as they are either a hard 0V or VCCb, correct?

Thanks,
Nick

0 Clemens Ladisch over 5 years ago in reply to Nicholas Carley

Guru 316690 points

All these pins are inputs.

The A/B pins can have their output drivers disabled (with DIR, OE, or V_CCX=0), but the input buffers are always active. So all these pins must be at GND or V_CCX.

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SN74AVC4T245: VCC isolation