LSF0108-Q1: Can I consider as an impedance of a transmission line?

Fabio Posser

Part Number: LSF0108-Q1
Other Parts Discussed in Thread: TXU0304-Q1

Hi, how should I consider this components in the transmission line?
I have an IC with series resistor and controlled 50ohms trace, then this shift level LSF0108-Q1 and the trace to the microcontroller.

Should I consider the On-state resistance as a series resistor on the transmission line? Or I need to consider this IC as a new driver with On-state resistance as output impedance and then place the series resistor termination?

Thank you.

over 1 year ago

+1 Clemens Ladisch over 1 year ago

Guru 263000 points

The LSF is a passive switch, so it pretty much behaves like a resistor. (The bond wires and on-die traces have a different, unknown impedance, but this cannot be avoided.)

0 Fabio Posser over 1 year ago in reply to Clemens Ladisch

Prodigy 165 points

Thank you Clemens.

0 Fabio Posser over 1 year ago in reply to Clemens Ladisch

Prodigy 165 points

What would be the suggestion of placement of it? I have a microcontroller, with SPI, that communicate with Wifi Chip.

I placed the series termination resistor near to the microcontroller.

And the shift level was placed near to the wifi module, due to the 1V8 power supply there.

But as it acts like a resistor, how should it be consider?
I have a series resistor -> long 100ohms track -> shifht level -> wifi module.

+1 Clemens Ladisch over 1 year ago in reply to Fabio Posser

Guru 263000 points

The LSF is typically not used for signals that must be so fast that transmission line effects matter. Anyway, putting it near one end should be OK.

For SPI, you'd typically use the TXU0304-Q1.

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LSF0108-Q1: Can I consider as an impedance of a transmission line?