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CD74ACT541 output pull down resistor

Mark Liang
Intellectual 740 points
Other Parts Discussed in Thread: CD74ACT541

Hi,

Could I connect the output of CD74ACT541 directly to a TTL input with out any pull up or pull down resistors?  If not, what are the recommended values for them?

Thanks.

  • 0
    TI__Guru* 96591 points

    Mark,

    The short answer is 'yes.' The long winded explanation follows.

    The CD74ACT541 has a push-pull output architecture, which means it will force a voltage on the output without the need for pull-up or -down resistors.  The output high voltage is referred to as V_OH and the low is V_OL.  If your system is running at 5 V (TTL), the V_OH for the CD74ACT541 has a minimum of 3.7 V (at some extreme temperatures/currents).  The minimum V_OH for the TTL standard is 2 V, so there will be no problem here.  The V_OL at 5 V for the CD74ACT541 has a maximum of 0.5 V (also under extreme conditions).  The TTL standard only requires 0.8 V max, so there is also no problem with V_OL.

    In less extreme circumstances (ie normal use - low current, room temperature), V_OH will be very close to 5 V, and V_OL will be very close to 0 V -- which gives even more headroom for the standard.

  • 0
    TI__Guru* 96591 points
    Mark - I realized after my post that the CD74ACT541 can also be put into a 'Hi-Z' state on the output. If you plan to use the OE function of the part, you should use a large pull-down resistor on the output (10k or 100k would work fine). This is just to ensure that the output remains low when a high impedance output is selected.
  • 0 in reply to Emrys Maier
    Intellectual 740 points
    Thank you very much!