HCT/TTL output current and supply current

[Grant Stockly]

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

 

[Lord Garth]

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?

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

 

[Phil Allison]

"Grant Stockly"

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.

** Says " Per Output Pin ".

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

It lists +/- Icc and +/- Ignd as 70mA.

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

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?

** 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

 

[Rich Grise]

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?

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

 

[Grant Stockly]

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

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