LSF0108: why the float level measured at A side is a little higher than vrefA?

MichaelK

Part Number: LSF0108
Other Parts Discussed in Thread: SN74LVC8T245

Hi,sir

We are using LSF0108 as a level shifter for 1.8 to 5V and 3.3V to 5V. Because our system A GPIO level is 1.8V and 3.3V, and system B GPIO level is 5V.

Now I have some questions need consult you

1. Do you need add pull up resistors on the low voltage side(in my system , 1.8 and 3.3V)?

2. How to config the GPIO pull-up or pull down internal resistors, does they have effect on the level shift?

3. Please see the attachment.

In the schematic, vrefA=1.8V, vrefB=5V, and pull-up resistors on B side is 850ohm.

all A side IO are floating.

What confused me is , Why the voltage is 2.1V when I measure the A1--A8.

In other words, why the level is a little higher then the vrefA?

over 7 years ago

0 ShreyasRao over 7 years ago

TI__Guru 62050 points

Hi MichaelK,

There is a video training series created on the LSF to specifically answer all the questions related to biasing, up and down translation, requirement of pullup and pulldown resistors here below.
training.ti.com/TLM-LSF-Intro

Please let me know if you have additional questions.

0 MichaelK over 7 years ago in reply to ShreyasRao

Intellectual 370 points

Hi,ShreyasRao

Thanks for your reply!

Please help to confirm if I need to add pull up resistors on the A side(lower voltage side)?

0 MichaelK over 7 years ago in reply to ShreyasRao

Intellectual 370 points

Add in our application, both sides are only GPIOs ,
Which one of the level shifter solution do you suggest to use, TXB,TXC, LSF, or others?

0 ShreyasRao over 7 years ago in reply to MichaelK

TI__Guru 62050 points

You do not need pullup resistors on the input side, if the signal is push pull. If the signal is open drain, you would need one to define the logic high state.
You could also consider using the SN74LVC8T245( for 1.8 to 5V DIR controlled translation ) device for GPIO translation assuming they are all push pull signals. The pullup resistors are not required either for translation in the case of SN74LVC8T245.

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LSF0108: why the float level measured at A side is a little higher than vrefA?