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LSF0204: LSF0204 Simulation with Pull-up Resistor and Using Push-Pull

Raju C
Prodigy 10 points
Part Number: LSF0204
Other Parts Discussed in Thread: LSF0101, TXU0202

Hi, I am using LSF0204 for up conversion and down conversion from 3.3V to 5V and Vice versa. I have connected UART lines, and as it is unidirectional So I am thinking there is no pull up resistors required as it will auto detect it as Push-Pull, is it right?

I got LSF0101 Pspice model and I had simulated this from 500KHz to 10MHz frequency, because of trace length I considered 50pF as load capacitance, and added 649E pull up resistor at the output line. Which is giving me 0.5V offset at the output. Is it because the resistor or because of the mismatch in the input for the internal FET ?

MCU side I have 16mA sink current, According to the datasheet pull up resistor table at this combination I suppose to get <3mA. 

When I changed the Pull up resistor from 649E to 2K , I am getting the result without offset and current has to be very less but in simulation it is showing 28mA

Can you please let me know if I am using UART , Does it auto detect or pull up resistors are required at output for stability?

And the chosen resistor is Okay or do I have to change it? 

Can you please tell me whether the attached results are correct?

why calculated current is not matching the results?LSF0101 SCHUp conversion with 649E pullup at outputUp conversion with 2k pullup at output

  • 0
    Guru 295240 points

    The LSF is just a passive switch. It does not detect anything; pull-up resistors are needed when the circuit is not able to generate a high voltage otherwise.

    Even with push/pull input signals, the LSF outputs usually need pull-up resistors. See the FAQ in the LSF application report.

    The device that drives an LSF input must be able to sink the current through the pull-up resistor. Higher currents will indeed increase the voltage.

    The table in the datasheet tells you what resistor to use for what current. But it does not tell you what current would be suitable. (You should use just enough current so that the rising edges are fast enough for your application.)

    The TXU0202 or TXU0x04 would be much easier to use.