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Science Forum Index » Electronics - Basics Forum » Maximum load current for 74HC device?...
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| Bill Bowden... |
 Posted: Tue May 20, 2008 5:05 pm |
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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 |
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| Bill Bowden... |
| Posted: Wed May 21, 2008 1:42 pm |
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On May 21, 6:40 am, John Fields <jfie... at (no spam) austininstruments.com> wrote:
Quote: On Tue, 20 May 2008 20:05:41 -0700 (PDT), Bill Bowden
wrongaddr... at (no spam) att.net> wrote:
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?
---
Depends on what you mean by "loaded".
For example, if four of the outputs were sourcing current while the
other four were sinking current they could each handle 12.5mA.
JF
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 |
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| Bill Bowden... |
| Posted: Fri May 23, 2008 3:25 pm |
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On May 22, 10:41 pm, "David L. Jones" <altz... at (no spam) gmail.com> wrote:
Quote: "Bill Bowden" <wrongaddr... at (no spam) att.net> wrote in message
news:6c10441c-c46f-461d-bb75-66d17d944d90 at (no spam) 25g2000hsx.googlegroups.com...
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
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.
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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