LSF0108: 3v3 to 1v8 and Pull up resistor is 300k

Jason Dai

Expert 3720 points

Part Number: LSF0108
Other Parts Discussed in Thread: SN74AUP1G17

Dear E2E,

Our customer is using LSF0108 to translate a reset signal from 3v3 to 1v8.

there are some reason that customer needs to use pullup 300k at 3v3 side.

the input side is at 3v3.

we didn't connect anything on 3v3 side and just a 300k pullup resistor to 3v3.

ideally, we should see 3v3 on input pin, but we only see around 2v there.

may i know the reason?

if customer still need to use 300k pullup, how to modify the circuit to fix the issue?

BR,

Jason

over 6 years ago

0 Clemens Ladisch over 6 years ago

Guru 325610 points

Something sinks a current of 4.3 µA. What else is connected to B1? How is the signal driven?

0 Jason Dai over 6 years ago in reply to Clemens Ladisch

TI__Expert 3720 points

Ladisch,

they are floating now.

in final system, the driven type is push pull.

BR,

Jason

0 Karan Kotadia over 6 years ago

TI__Genius 16735 points

Hi Jason,

Please review the following video series on our LSF translation device. https://training.ti.com/down-translation-lsf-family?cu=1134826

I would highly recommend adding a bias capacitor to the enable pin. This might be causing some issue.

Are the pullup resistors really required? The video I added above will help answer that question.

Are you always down translating? Is the translation always unidirectional?

You might be able to use another part that isn't as complicated for your design.

Thanks!

-Karan

0 Jason Dai over 6 years ago in reply to Karan Kotadia

TI__Expert 3720 points

Karan,

Yes, the pullup resistor 300k is required and the translation is always unidirectional from 3v3 to 1v8.

the type of 3v3 driver is open drain.

could you please recommend a suitable device in this case?

BR,

Jason

0 Clemens Ladisch over 6 years ago in reply to Jason Dai

Guru 325610 points

For unidirectional downtranslation, you can use any buffer with an overvoltage-tolerant input, e.g., SN74AUP1G17.

0 Karan Kotadia over 6 years ago in reply to Clemens Ladisch

TI__Genius 16735 points

Thanks for your inputs clemens!

-Karan

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LSF0108: 3v3 to 1v8 and Pull up resistor is 300k