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SN74LVCH244A Input Current

Using a SN74LCVH244A and have a question pertaining to the input current.  Ii on the datasheet states +-5uA max. If the input is either going to be a GND or a pull up to 3.3V, does this imply that the pull up resistor is very large, given that the input current is very small? 

R = 3.3V / (5 x 10^-6A) = 660K resistor. Does this seem correct?

  • Hi Anthony

    You are correct. However I would not run at the borderline. So guardband that number a little.

    Also if  you are a Ti'er you should post on the int logic forum.

  • Ok, makes sense. I would have taken advantage of the bus hold circuitry, however I need the output of the buffer to either be 0V or 3.3V, since the load is a 4.7K resistor pulled up to 3.3V.

  • Anthony

     I did not notice the part was a bus hold part. The bus hold will hold a floating input to either Vcc or gnd.  You need to be very careful using a pullup resistor with a bus hold part.

    Pullups resistors are not nessessary but if you do use one then you will need to have enough current to overdrive the bus hold. In this case it is 500ua specd in the bus hold specs..

    See attached appnote

    Bus hold appnote.pdf
  • Chris,

    The reason why I ask this is because I am coming across designs that have CMOS inputs pulled up to 3.3V with resistors in the range from 330Ohm to 4.7K.  This would give the CMOS input milliamps, not sure if there is a direct implication for this. For instance, is there an absolute max input current on the 244?  Is this what the input clamp current is?  I guess mA is fine if they were TTL chips, but in CMOS I presume this is too much. 

    So, to overcome the bus hold circuit, greater than 500uA is needed, but not to exceed 505uA then? since the max current is +-5uA.  Still confused about that. Is the amount over 500uA that would be sent to the inputs?

  • The clamp current is the limit of the clamp diode and will not apply unless you are going above Vcc or below gnd.

    The Ii current is the leakage current. This means you need at least 5ua to drive the part.

    There will be 2 specs related to bus hold. The lower number 75ua is the minimum amout of current the part is guarranteed to use to drive the input high or low.

    The higher number 500ua is the minimum amount of current you should use to overdrive bus hold.  So if you are using pullup resistors they should pull 505ua (Bushold + Ii) or greater to overdrive bus hold and toggle the part.

    The part will only use the current it needs to drive the input. there is no max becuase it will only use what it needs.

    If the sockets have pullup resistors then there is no need for a bus hold part. If they have a bus hold part then there is no need for pullup resistors.

    However In some cases they power down part of a circuit or they other part on the line needs a pullup,.  In these cases they can use pullup resistors with a  bus hold part  but it must be strong enough to overdrive bus hold. 

  • Great, that answers all of my concerns. Thanks for your time.