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SN74LVC1G00: MAXIMUM recommended PULLUP resistor values for LVC logic INPUTS

Robert Buono
Prodigy 10 points
Part Number: SN74LVC1G00

Typically pullup resistor values for logic inputs are in the range of 4.7K ohm to 10K ohm.

However, there are some situations where I would like to use a higher value. 

This is usually to save power, or when the driving device does not have high enough sinking current ability.

Can you please recommend a MAXIMUM pullup resistor value that can be used with LVC inputs?

i.e. can 100K ohm be used?

NOTE: If there are different constraints due to DC input characteristics as compared to input rise time, please differentiate these in your response.

  • 0
    TI__Mastermind 18360 points

    Hi Robert,

    This FAQ, How do I size pull-up or pull-down resistors?, will go into detail on this topic. Let me know if you have any further questions. 

    Regards,

    Sebastian 

  • 0 in reply to Sebastian Muriel
    Prodigy 10 points

    Hi Sebastian,

    So... looking at an LVC1G58 datasheet, I see IsubI spec of +/-1uA (I believe that is input leakage current.  If I am wrong please point me to input leakage current in the datasheet).  So, if I was worried only about DC drop, and wanted to limit it to say 0.5V, then I could use a pullup resistor value as high as 500K ohm.  However, if I need to meet the 20V/nsec slew rate requirement at the input, this is quite an onerous requirement.  If we assume an input capacitance of 15pF, then this requires a pullup CURRENT of 750uA AT THE THRESHOLD SWITCHING LEVEL.  So lets assume that is 2.5V for a Vcc of 5V.  That would mean a pullup resistor value of 2.5V/750uA = 3.3K ohm or less would be required.  

    ALTERNATIVELY... is it fair to say that if I use only LVC parts with SCHMITT-TRIGGER INPUTS, the input slew rate requirement is no longer needed.  Much slower input slew rate can be used and still have clean output switching.  In this case would I ONLY have to worry about the DC input leakage current restrictions, and could therefore use a pullup resistor value as high as 500K ohm, as described above?

    Thank you! 

  • 0 in reply to Robert Buono
    TI__Mastermind 18360 points

    Hi Robert,

    Robert Buono said:
    +/-1uA (I believe that is input leakage current

    The LVC1G58 max input leakage current is listed under the Electrical Characteristics II = +/- 1uA. Keep in mind the max input leakage current for the LVC1G00 is +/- 5uA. To limit the drop across the resistor to 0.5V for the LVC1G00, you would need a 100k ohm pull-up. This would not satisfy the devices transition rate requirement however. 

    Robert Buono said:
    However, if I need to meet the 20V/nsec slew rate requirement at the input, this is quite an onerous requirement.  If we assume an input capacitance of 15pF, then this requires a pullup CURRENT of 750uA AT THE THRESHOLD SWITCHING LEVEL. 

    Can you clarify where you got 750uA from? 

    Robert Buono said:
    is it fair to say that if I use only LVC parts with SCHMITT-TRIGGER INPUTS, the input slew rate requirement is no longer needed. 

    Yes, a device with Schmitt-Trigger inputs would allow for slow inputs and still have clean output switching. 

    Robert Buono said:
    In this case would I ONLY have to worry about the DC input leakage current restrictions, and could therefore use a pullup resistor value as high as 500K ohm, as described above?

    Yes you could use a 500k pull up resistor as long as the power supply can source that current. 

    Regards,

    Sebastian