HCT/TTL output current and supply current
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Posted by Grant Stockly on October 25, 2007, 3:30 am
Please log in for more thread options 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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Posted by Lord Garth on October 25, 2007, 4:16 am
Please log in for more thread options TTL is a current sinking family. Outputs sink current from an external source. | ||||||||||||||||
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Posted by Phil Allison on October 25, 2007, 9:06 am
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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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Posted by Rich Grise on October 25, 2007, 4:32 pm
Please log in for more thread options 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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Posted by Grant Stockly on October 25, 2007, 7:39 pm
Please log in for more thread options > 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 | ||||||||||||||||
>
> 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?
>