LSF0204: Down translation from A to B

Wayne Eads

Part Number: LSF0204

Can down translation work from port A to port B? I have my circuit configured in such a way (5 -> 3.3). From supplemental material, such as the YouTube video, it seems clear that this is not the recommended means of down conversion. However, I didn't gather that from the datasheet and treated the device as any other translator. Without pull-up resistors, logic highs on the B side are 2.4 volts instead of the desired 3.3 volts. Is there a way to salvage what I have with the correct pull-ups for a prototype unit? Or should I simply start over with the proper configuration?

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 317250 points

Vref_A must be no higher than the lowest signal voltage, 3.3 V. Is this the case in your circuit?

When down-translating from B to A, voltages at the A pins are clamped by Vref_A. Otherwise, the output voltages are determined only by the pull-up resistors.

0 Wayne Eads over 4 years ago in reply to Clemens Ladisch

Prodigy 25 points

No, my Vref_A is 5 volts. I understand that down translation is supposed to happen from B to A, but with an A to B implementation, is there any way to salvage this hardware? Should correct pull-up resistors (200 ohms) pull the levels up 2.4 volts to 3.3 volts?

0 Clemens Ladisch over 4 years ago in reply to Wayne Eads

Guru 317250 points

No, you can use any signal voltage at the I/O pins. Only the reference pins have the restriction Vref_A + 0.8 V < Vref_B.

With Vref_A > Vref_B, the internal transistors are not properly biased.

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LSF0204: Down translation from A to B