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Sci.Electronics.Basics -> HCT/TTL output current and supply current
There are 5 messages in this thread.
You are currently looking at messages 1 to 5.
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Author: Grant StocklyDate: 03:30 25-10-07
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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
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Author: Lord GarthDate: 04:16 25-10-07
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"Grant Stockly" <grant@stockly.com> wrote in message
news:1193297404.621164.300260@e34g2000pro.googlegroups.com...
> 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.
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Author: Phil AllisonDate: 09:06 25-10-07
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"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
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Author: Rich GriseDate: 16:32 25-10-07
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On Thu, 25 Oct 2007 00:30:04 -0700, Grant Stockly wrote:
> 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
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Author: Grant StocklyDate: 19:39 25-10-07
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On Oct 25, 12:32 pm, Rich Grise <r...@example.net> wrote:
> On Thu, 25 Oct 2007 00:30:04 -0700, Grant Stockly wrote:
> > 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
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
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