LSF0204: pull up resistor

Mason Noh

Expert 2490 points

Part Number: LSF0204
Other Parts Discussed in Thread: SN74AVC4T774, TXB0104,

Hi TI experts,

I have tested level shifter for SPI(25Mhz).

The datasheet shows the calculation formula for the pull-up resistance.

But it is not working even by applying a calculated value. Even applying the reference values does not work.

In some cases VLO is too large, and in others the input is output as it is.

So I figured out the resistors through trial and error.

R247, R248, R250: open

R249: 1Kohm

R251,R253,R254: 294ohm

R252: open

Below waveform show very long rising time. How can fix it. If I reduce pull-up resistance, VLO is increase.

ch1: B1(pin10)

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 243000 points

The LSF uses passive switches. When you're using a small pull-up resistors, the external device must be able to sink all the current that flows through those resistors.

In this application, it would be a better idea to use a direction-controlled translator with active buffering, such as the SN74AVC4T774.

0 ShreyasRao over 4 years ago

TI__Guru 55960 points

Thats the frequency vs load tradeoff on the LSF. High frequency requires small value of pullup resistors.

Low freq can have higher value of pullup resistors. In order to have fast edge rates, requires small PU resistors as well.

0 Mason Noh over 4 years ago in reply to Clemens Ladisch

Expert 2490 points

Hi Clemens,

It's very difficult to change another parts because of MP schedule.

Is it impassible to solve this issue using pull-up resistor?

0 Clemens Ladisch over 4 years ago in reply to Mason Noh

Guru 243000 points

You have to find values for the pull-up resistors that balance V_OL and speed to whatever is acceptable for your application. If that is not possible, then you cannot use the LSF.

0 ShreyasRao over 4 years ago in reply to Mason Noh

TI__Guru 55960 points

Mason,

The Vol is 0.5V, and it should be acceptable for a 3.3V logic. Also, I see that the rise time is about ~10ns which should be acceptable for the CMOS logic devices which usually suggest 10ns/V to 20ns/V. So, for 3.3V case, the rise time expected is about 30ns to 60ns.

0 Mason Noh over 4 years ago in reply to Clemens Ladisch

Expert 2490 points

Hi Clemens,

Thank you for your reply.

How about TXB0104? It is pin compatible with LSF0204.

Is it can be a solution?

0 ShreyasRao over 4 years ago in reply to Mason Noh

TI__Guru 55960 points

Hi Mason,

You could consider the TXB0304RUT for 1.8 to 3.3V.

Please remove the external pullup resistors from the schematic.

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LSF0204: pull up resistor