# HCT/TTL output current and supply current

Discussion in 'Electronic Basics' started by Grant Stockly, Oct 25, 2007.

1. ### Grant StocklyGuest

I am trying to compare some standard series and LS parts to HCT parts.

I don't understand how the maximum ratings are calculated.

For example, the Phillips data sheet for the 74HCT125 lists +/- Io at
35mA. It lists +/- Icc and +/- Ignd as 70mA.

Then it lists the power dissipation per package at 750mW. If the
maximum current for Icc or Ignd is 70mA, how do you get 750mW for a
maximum supply voltage of 7V?

Grant

2. ### Lord GarthGuest

TTL is a current sinking family. Outputs sink current from an external
source.

3. ### Phil AllisonGuest

"Grant Stockly"

** Says " Per Output Pin ".

So 140mA is possible with all four outputs in use.

** Cos something in the common Vcc & Ignd lines will not tolerate 140 mA
indefinitely.

** 140mA at 7 volts = 980mW.

The max dissipation figure ( 750mW) is * theoretically * well possible under
overload conditions - assuming the full 7 volts is dropped across the
conducting MOSFETS.

....... Phil

4. ### Rich GriseGuest

You use some care in your design so that the total dissipation is
less than 750 mW, like don't load them to the limit. A TTL input
shouldn't need more than a few mA to get pulled high (although, I'd
use a series resistor to limit _base_ current), and when pulled low,
it only draws leakage, which will^H^H^H^Hshould be negligible.

If you're trying to use them for power drivers, then, as I said,
use care in your design, or use ULN2003 or so power buffers.

Hope This Helps!
Rich

5. ### Grant StocklyGuest

Thanks for the advice that everyone has given me. I am trying to
choose a modern part to replace some bus drivers on a new S-100 card
design. I'm trying to find a suitable replacement for the 8T97/74367
and a new implementation using the '125 parts. I can get old parts
but they are not as reliable as the new ones it seems.

The HCT parts look like they can drive enough TTL loads to be a good
match for the project. I don't expect that any of the pins will see
any huge amount of current except for one, but that won't happen very
long or very frequently.

Grant