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LSF0204D: Pull down on low side signal

Deborah Calabrese
Prodigy 20 points
Part Number: LSF0204D
Other Parts Discussed in Thread: SN74LVC125A

Hello,

I'm using an LSF0204D to convert signals from 3v3 to 1v8.

I have a push-pull transmitter and the signal needs always to be down translated.

The signal on the high side has not a pull up while the signal on the low side needs a pull down. 

I'm using a 4.7k resistor as pull down.

Can this pull down cause any kind of issue when I drive the low side input high?

Thanks for your advise

  • 0
    Guru 316920 points

    The LSF is a passive switch and needs pull-up resistors to generate the high-level output voltage. With a pull-down, you would never get a high-level voltage of 1.8 V out of it.

    For unidirectional downtranslation, you can simply use a buffer with overvoltage-tolerant inputs, such as the SN74LVC125A. A proper buffer can handle pullups or pulldowns at its inputs and/or outputs.

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Hello Clemens,

    thanks for your answer.

    When you say ' With a pull-down, you would never get a high-level voltage of 1.8 V out of it ' you mean that I can't get a strong 1.8V but only a weak one? Please, correct me if I understood wrong.

    Having only an nMOS pass gate I can reach just a weak '1', and in fact I measure 1.64V, when I try to drive high the low side signal.

    I have also seen from your LSF documentation ' Voltage-Level Translation With the LSF Family' that you suggest to have only high impedence loads during the weak '1' to avoid that current will cause the output voltage to drop significantly.

    Please let me know if there also other drawback.

    Thanks for your advise

  • 0 in reply to Deborah Calabrese
    Guru 316920 points

    The low-side high signal is driven only by the current through the transistor. Driving a higher current would require the transistor to be more open, which can happen only if the output voltage is a larger amount below the gate voltage.

    The pull-down resistor should be 200 kΩ or so. And this would make the circuit very sensitive to noise.