# Maximum load current for 74HC device?

• posted

How much total current can be drawn from a typical 74HC device? I see specs that state the ground current cannot exceed 50mA and other specs that say the device dissipation is limited to 750mW. Most of the specs say that 25mA can be had from any one output, but it's hard to figure out the total for several outputs.

For example, the 74HC164 shift register is speced at 25mA per output pin, but only a total of 50mA for the ground pin. Does that mean the maximum output current for any one pin is limited to 6mA if all 8 outputs are loaded at the same time?

-Bill

• posted

Sure looks like that is correct. BTW, those numbers are the same across all of the HC line.

+/- 25mA / Pin +/- 50mA / Device
• posted

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Depends on what you mean by "loaded".

For example, if four of the outputs were sourcing current while the```
• posted

Yes, good point. I was thinking the sink and source limits were different, but it looks like they are the same for the 74HC164 and maybe other HC devices.

-Bill

• posted

Depends what you are driving. Those specs are usually referenced to maintaining CMOS compatible voltage levels. If you are driving a load that doesn't care about such things then you can get more.

Dave.

• posted

Yes, I think the total power dissipation is another factor. So, if you load an output where the voltage falls 50% at say 30mA (supply voltage

5), the dissipation will be 2.5*0.03 = 75 milliwatts. Doing that for 8 outputs would be 600mW total, which may be near the limit. I haven't looked at the max dissipation specs, but I think it's around 700mW. A little 14 pin DIP IC package will get fairly hot at 700mW dissipation.

-Bill

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