CMOS - TTL comparison + datasheet assistance

Discussion in 'Homework Help' started by Random3s, Nov 17, 2014.

  1. Random3s

    Thread Starter Member

    Jul 30, 2014
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    Hi all,

    I am currently completing a report comparing CMOS and TTL IC's. I am comparing a hex inverter IC, and need some assistance locating the output current (both high and low), of the CMOS hex inverter (CD74HC04).

    I understand there is a max amount of current a device can sink or source whilst still maintaining a high or low logic level, so generally a max source/sink current is designated per output pin....

    The datasheet is located here and I am comparing the CD74HC04 model (CMOS load only).

    I can see the max rating state:

    DC Output Source or Sink Current per Output Pin, IO
    For VO > -0.5V or VO < VCC + 0.5V . = .±25mA

    So as long as Vo is between than -0.5v and Vcc +0.5 the output pin is able to source and sink ±25mA per output?? So the high level output voltage for CMOS loads is 0.02A... meaning that the normal operating specs, output current is generally around 20mA per output pin?

    Id appreciate a shove in the right direction

    Thanks
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    Close. The table states that for the particular Vcc supply voltages of 2V, 4.5V, and 6V, the output will source or sink 20mA when the output voltage is within (Vcc-0.1V) or within 0.1V of ground.
     
  3. Random3s

    Thread Starter Member

    Jul 30, 2014
    38
    0
    Got it, thanks for taking the time to help me.
     
  4. RamaD

    Active Member

    Dec 4, 2009
    254
    33
    In 74HC series, the CD series from TI (earlier with Harris) have better drive currents of the order of 25mA. Whereas the M74HC from ST, MM74HC from Fairchild, SN74HC from Ti, are all much lower, approximately 5mA source/sink for about 0.5V drop, varying between makes! That way CD74HC series may not be a true representation of 74HC series.
     
  5. Random3s

    Thread Starter Member

    Jul 30, 2014
    38
    0
    Or actually in this case 20uA right, the sheet states 0.02mA.

    datasheet
     
  6. Random3s

    Thread Starter Member

    Jul 30, 2014
    38
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    Thats good information, thanks.

    I am comparing the 74LS04 I can see that it has a Iil of 0.4ma compared with the CD74HC04 which has an Iil of 0.02ma....
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    Yes, it's .02mA of course, not 20mA (.02A).
    For higher current outputs, look at the TTL load values.
     
  8. Random3s

    Thread Starter Member

    Jul 30, 2014
    38
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    I am still a little confused by this to be honest. As I read the data sheet, the .02mA figure is purely a test condition. As only an absolute maximum is provided on the data sheet are we to presume that the outputs can handle a max of 25mA, but we should be aiming at lower than this in order to avoid damaging the IC?

    DC Output Source or Sink Current per Output Pin, IO
    For VO > -0.5V or VO < VCC + 0.5V . . . . . . . . . . . . . . . . . . . .±25mA
     
  9. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    The large the output current(Ioh) the large the voltage droop across MOS. And this will reduce Voh voltage.
    And if Voh drops below Vih, the next gate will not properly recognize a high state at his inputs.
    hc_voh.jpg
     
  10. Random3s

    Thread Starter Member

    Jul 30, 2014
    38
    0
    Got it, thanks Jony.
     
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