TXB0304: open drain scenario

David Zhao

Part Number: TXB0304
Other Parts Discussed in Thread: LSF0204

Hi Supporter

I know that TXB0304 datasheet shows it can only translate push-pull logic. But my customer adopt this device for open drain output and the board already manufactured. Schematic is as below. Need to translate left 1.8V(open darin) logic to right 3.3V logic. How to support this scenario? Or is there pin to pin solution to support this scenario? Thanks!

Regard

David

over 7 years ago

0 ShreyasRao over 7 years ago

TI__Guru 61570 points

Hi David,

the schematic shown above doesnt have a TI device on it.

I am unable to comment on the performance of comp device.

0 ShreyasRao over 7 years ago in reply to ShreyasRao

TI__Guru 61570 points

www.ti.com/.../scea054a.pdf

I will generally explain the theory here for why we recommend TXS for OD interface.

we recommend not to use TXB for OD devices because they require pullup resistors to define a high level.
Since TXB internally has 1K series resistors, it will form a voltage divider and reduce the output voltage.
As long as the customer uses high enough pullup resistor to not cause any issues with voltage levels (Voh or Vol), it can be used.

0 David Zhao over 7 years ago in reply to ShreyasRao

TI__Intellectual 1820 points

Hi Shreyas

Sorry for attach the wrong schematic. Please find below updated. Current left pull up resister is 470 ohm and right pull up resister is 22.1 kohm. How do you recommend the value of pull up resister? Thanks!

Regards

David

0 ShreyasRao over 7 years ago in reply to David Zhao

TI__Guru 61570 points

Hi David,

Please refer to my earlier post and refer to the app note regarding the pullup resistor on TXB.
470 ohm is very strong pullup which I believe will cause a problem for maintaining the low value.
You can remove pullups on one side of the device ( output) and only have at the input (suggested 22k).

0 David Zhao over 7 years ago in reply to ShreyasRao

TI__Intellectual 1820 points

Hi Shreyas

In fact, with 470 ohm pull up, i see oscillate issue as below. Is this cased by very strong pull up or other reason? Thanks!

Regards

David

0 ShreyasRao over 7 years ago in reply to David Zhao

TI__Guru 61570 points

David,

Is this the output? I would not think that strong pullup can cause this.

0 David Zhao over 7 years ago in reply to ShreyasRao

TI__Intellectual 1820 points

Hi Shreyas

This is 470 ohm pull up side input. Now 22.1kohm resister is removed. What do you think can cause this input oscillate? Thanks!

Regards
David

0 ShreyasRao over 7 years ago in reply to David Zhao

TI__Guru 61570 points

Please replace 470 ohm to atleast 10k to see if anything changes.
22K resistor removal is good as it is redundant ( TXB is push pull)

0 David Zhao over 7 years ago in reply to ShreyasRao

TI__Intellectual 1820 points

Hi Shreyas

In fact, it is the output oscillate first and the input follow oscillate as below waveform shows. Input pull up resister change to 10k doesn't solve this issue. Is there any OD solution that is pin compatible with TXB0304 that customer can use? Thanks!

Regards

David

0 ShreyasRao over 7 years ago in reply to David Zhao

TI__Guru 61570 points

Hi David,

I see that the changing of pullup resistor helped here as I compare against the previous scopeshot you provided. Maybe having a higher size will benefit further in limiting the ringing.

Unfortunately, I had checked if there was a replacement TXS device when you posted your query first time, however, we do not have TXS to directly replace the TXB0304 device neither in the RUT or RSV package.
There is LSF0204 in RUT package which can work , it could be tried here.

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TXB0304: open drain scenario