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LSF0102: Pull up resistor

Jeff YANG
Mastermind 7930 points
Part Number: LSF0102
Other Parts Discussed in Thread: TXU0102, TXU0202

Tool/software:

Hi,

I pulled off the 300 ohm pullup resistors on the input and output and was then able to drive the input to 0v (otherwise I could only drive it to 500 mV).  Without the pullup resistors, however, the output will only drive up to 1.5V.

 I’m working on bringing up the new Auto IOB and came across what I think is a hardware issue on the level translators.  In the schematic captures below, there are 6 level translators and I found with the CAN interface that I can’t drive the STB low on pin 3 or 4 of U18.  It looks like for this particular device there might need to be two different resistor values on each side – perhaps a 1550 ohm on the 5V side and a 280 ohm on the 1.2V side?

Thanks!

Jeff

  • +1
    Guru 317490 points

    You do not need pull-up resistors on inputs pins that are driven with push/pull signals.

    You do need pull-up resistors on the outputs pins. When translating to a lower voltage, 10 kΩ suffices. When translating to a higher voltage, the resistor value determines the speed of the rising edge because the resistor and the trace/device capacitance form a low-pass filter; 300 Ω is likely to be much too low. When in doubt, check the waveform with an oscilloscope.

    For unidirectional signals, unidirectional translators like the TXU0102/TXU0202 would be easer to use and more robust.