SN74ABT16245A false drive issue in -40C

Randy Wang

Hi Team

My customer is using 74ABT16245 for LED fault indicator, the input is from OPTO iso signal, the output is to drive LED.

It's normal in normal temperature, but in -40C, the input is low, but the PIN17 (16245A 2B4) has 3.3V voltage, so the output is high, remove the IC it is low again, it seems the input of 16245 force to high.

Could you help to review the schematic and give us some comments? thanks.

over 8 years ago

0 Clemens Ladisch over 8 years ago

Guru 316690 points

The bus transceiver requires an input transition rate of no more than 10 ns/V. But even when ignoring the fall time of the optoisolator, RN106 and C136 form an RC filter with a time constant of 940 µs.

Anyway, this problem sounds as if the DIR inputs are not handled correctly. What is the voltage drop over R701/R702?

0 Randy Wang over 8 years ago in reply to Clemens Ladisch

TI__Intellectual 1705 points

Hi Clemens
Thanks for your replay.
I didn't figure why this issue connect to DIR? Is the R701, R702 resistance issue?
What's your suggestions about the RC parameters?

0 Clemens Ladisch over 8 years ago in reply to Randy Wang

Guru 316690 points

3.3 V looks like the high level of a TTL output. One possible explanation is that the current through R701/R702 is too high, so that DIR are read as high.

What is the voltage at 2A3?

I guess the RC filter is there for a reason. Anyway, removing it would not reduce the fall time of the optoisolator; I think you need a Schmitt trigger.

0 Randy Wang over 8 years ago in reply to Clemens Ladisch

TI__Intellectual 1705 points

So I'll try to verify two things:
1. short R701/R702;
2. remove C I think it's enough right?

0 Clemens Ladisch over 8 years ago in reply to Randy Wang

Guru 316690 points

I'm just guessing; this is what I would try first.

0 ShreyasRao over 8 years ago in reply to Clemens Ladisch

TI__Guru 62050 points

Hi Clemens ,

Thanks for your inputs. I would be curious to know this as well . That under normal ambient temp it is fine and issue is seen at -40C is strange.
what is temp tolerance for the R and C ? Please ensure that the input transition rate is not violated as mentioned by Clemens.
False triggering is an issue if tr is not adhered to. Schmitt trigger buffer is an option if remove of the 0.1uF doesn't help.

0 Randy Wang over 8 years ago in reply to ShreyasRao

TI__Intellectual 1705 points

Hi Shreyas

I still didn't get it why I need schmitt trigger buffer? and any ideas how to modify on customer board? thanks.

0 ShreyasRao over 8 years ago in reply to Randy Wang

TI__Guru 62050 points

Hi Randy ,

Schmitt trigger device can handle slow rising edge inputs. This is true in your case with the RC setup in there .

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SN74ABT16245A false drive issue in -40C