LSF0108: Some questions of LSF0108 about the pull up and pull down resistance

user6199951

Part Number: LSF0108
Other Parts Discussed in Thread: TXB0108, SN74AXC4T774

i want to conversion the SPI signals' s level from 1.2V to 3.3V, the LSF0108 requires both 1.2V and 3.3v need to pull up, but the spi_EN need to pull down and connect to GND ,how should i do to meet the requirements of conversion the spi_EN's level and pull down to GND at the same time.

Meanwhile, can i also need to make other spi signals pull up to 1.2V or 3.3V use the 4.7K resistance when i alreaday pull up them to 1.2V and 3.3V in the lsf0108 side ?

Thanks very much.

over 5 years ago

0 Clemens Ladisch over 5 years ago

Guru 316690 points

LSF devices require pull-up resistors.

SPI signals are unidirectional; the best solution for this would be the SN74ACX4T774.

Why are you using an eight-channel device?

0 user6199951 over 5 years ago in reply to Clemens Ladisch

Prodigy 100 points

Because i also need to convert the other signals

0 Clemens Ladisch over 5 years ago in reply to user6199951

Guru 316690 points

What other signals? The electrical characteristics of those signals affect which translator device can be used.

0 user6199951 over 5 years ago in reply to Clemens Ladisch

Prodigy 100 points

sorry i just read it wrong , i need a unidirectional translator device , and the LSF0108 is a bidirectional translator,so i select it.

Also we've alreaday bought the LSF0108, so i need your help to solve the questions what i ask above, thank you very much.

0 user6199951 over 5 years ago in reply to Clemens Ladisch

Prodigy 100 points

Hi,,are you still online?

i really need your help, it's urgent.

Thanks very much!

0 Clemens Ladisch over 5 years ago in reply to user6199951

Guru 316690 points

It is not possible to use a pull-down with the LSF.

0 user6199951 over 5 years ago in reply to Clemens Ladisch

Prodigy 100 points

Ok, and the other question,can i also need to pull up signals with 4.7K resistence when i've pulled up them beside LSF0108.

0 Clemens Ladisch over 5 years ago in reply to user6199951

Guru 316690 points

Increasing the pull-up resistance reduces the possible speed. Check with an oscilloscope how far you can go.

0 Karan Kotadia over 5 years ago in reply to Clemens Ladisch

TI__Genius 16735 points

Hi,

I recommend you watch the following video series on the LSF device to become an expert on the part. (It is very short)

https://training.ti.com/introduction-voltage-level-translation-lsf-family?context=1134826-1139264-1134789

Thanks!

-Karan

0 user6199951 over 5 years ago in reply to Clemens Ladisch

Prodigy 100 points

Thanks a lot about your answer,

And how about TXB0108,can i use it transmit SPI signals? The SPI_CLK need to be pull down at the initial state, and then pull up.

0 Clemens Ladisch over 5 years ago in reply to user6199951

Guru 316690 points

The TXB is capable of transmitting SPI signals, but does not handle all loads or pull-up/-down resistors; see sections 8.3.3 and 8.3.5 of the datasheet.

All auto-bidirectional level shifters have restrictions; for unidirectional signals like SPI, a direction-controlled level shifter like the SN74AXC4T774 is a much better solution.

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LSF0108: Some questions of LSF0108 about the pull up and pull down resistance