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questions about LSF0108 pull up resistor

Peter Chen(ICP_FAE)
Intellectual 2480 points
Other Parts Discussed in Thread: LSF0108

Some questions about the LSF0108.

According to figure 10.2.3, some paths need install the pull up resistor and some does not.

1) Is the pull up resistor must required in input/output of LSF0108?

2) If not, what kind of scenarios need pull up resistor in input and output?( ex: only the open drain topology need pull up?).

3) What is the proper pull up resistor  value?

  • 0
    TI__Genius 9860 points

    Hi Peter, section 5.2 of this app note explains when pullup resistors are needed: http://www.ti.com/lit/pdf/slva675

    Regarding the size of the pullups, the datasheet gives some guidelines. However, choosing a pullup resistor size is a balancing act of what your system can tolerate in terms of power consumption, signal frequency, and current-sinking capability of the source driver.

    . In general, a smaller pullup resistor will allow the LSF to translate faster because of a smaller RC time constant on the signal line. However, a smaller pullup resistor will also draw more current from its pull-up voltage source, so you need to make sure that your signal driver can sink the proper amount of current.

    For low frequencies <1 MHz, pullups in the range of 1-5 k ohms are acceptable, while high frequency signals may need 300 ohms or as low as 150 ohms.