LSF0108: Usage without pull-up resitors for 3.3V LVCMOS to 5V TTL translation

luisb

Part Number: LSF0108

Hi,

I'm currently working on a design where I need to connect a 5V TTL interface to the 3.3V output of a FPGA. Because of it's small size I'm looking into using the LSF0108.
My Interface has to work bi-directional at a frequency of a few MHz (up to about 10). Since board area is strictly limited and as the 5V side is TTL (I only need 2.4V as logic high) I'm thinking about not using any pull-up resistors at all.

Am I correct in the understanding that the output side of the LSF follows the input side up to about Va?

I'd like to know if the following assumptions are correct.

Vb on Bn -> ~Va on An (This is allowed without any pull-up resistors according to your datasheet)

Va on An -> ~Va on Bn (I couldn't find any information on this. Usage as if it was something like a CB3T)

With Vb = 5V and Va = 3.3V

Thanks

Luis

over 6 years ago

0 Emrys Maier over 6 years ago

TI__Guru* 96957 points

Hi Luis and welcome to the forums!

I have asked our translation expert to get back with you.

Also, you might want to check out this video series that goes into detail on how the LSF family of translators works:

Understanding the LSF Family of Bidirectional, Multi-Voltage Level Translators

0 ShreyasRao over 6 years ago

TI__Guru 59720 points

Luis,

You could use it without pullup resistors on either sides and there wouldn't be any translation.
The output will be around 3.3V. It will decrease as the resistive loading increases (leakages).
Would you prefer this configuration?

Let me know if you have questions.

0 luisb over 6 years ago in reply to ShreyasRao

Prodigy 20 points

Thank you
This is exactly what I needed to know. Since there are Tri-State-Drivers on both sides and the distance is quite small, I think I will use this configuration.

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LSF0108: Usage without pull-up resitors for 3.3V LVCMOS to 5V TTL translation