CMOS ICs

Discussion in 'General Electronics Chat' started by JohnnyD, Sep 2, 2006.

  1. JohnnyD

    Thread Starter Well-Known Member

    Aug 29, 2006
    79
    0
    I am a hobbyist but have learned a great deal recently, to the extent that i am now designing circuits with ICs in.

    My question is this - in all the data sheets I've seen for the ICs I've used (4013, 4093) it says the maximum current for the inputs is 10mA. but it doesn't state a minimum. I'm trying to design automobile circuits to draw as little current as possible so I'd like to know how low I can reliably go.

    Also, when it says to ground any unused inputs, if i connect them directly to ground, they sink/source around 10mA. Is it OK to use an appropriate resistor to minimise current draw from unused inputs/outputs.

    Thanks.

    John.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    The +/-10 milliamp maximum figure is the DC current that is drawn by the device from the Power Supply. The current drawn by the signal inputs is measured in microamps.

    As for the question on whether to use a pullup or pulldown resistor on unused inputs to tie the input to power or ground, it is not necessary but you will find that many designs use them nevertheless. It is more of a convenience to use a resistor as it facilitates board testing since it permits an automated board tester to drive the input during a test of the board.

    Also if you avoid connecting the pin directly to power or ground you can then use the input in some future configuration of the board by simply removing the resistor which frees up the input for use.

    hgmjr
     
  3. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    CMOS draws only a few microamps if the outputs are not sinking or sourcing current, so they are Ideal for low current apps.
    The inputs can be connected direct to ground (or V+) with no problem, as they have a very high input impedence..(which is one reason why they are suseptable to static discharge from improper handling). The outputs can't though, and current must be limited from the output.
     
  4. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    All CMOS inputs are protected by internal diodes which only allow the input pins to got 0.7V above or below the supply rails. The 10mA quoted is the maximum that the diodes can pass. Where inputs can potentially go outside the rail voltages, it is wise to use a resistor in series with the input to limit the current through the diodes.
     
  5. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    143
    I think the configuration pebe is describing is something to the effect of this attachment. This simple circuit (and variants of) is used for ESD-protection for CMOS circuits.

    Dave
     
  6. s. pandey

    New Member

    Oct 12, 2006
    2
    0
    hi can i get some information about cmos layout design rules
     
Loading...