Minor correction. 10 mA * 1V = 10 mW    8 pins makes
for 80 mW.  Still not enough to worry about.

If you tie the LED high and light it by driving the
output low don't you get the higher differential
voltage (and thus the higher wattage)?  I chose to tie
low and drive high (no personal comments, please!).
the outputs were at around 4.5 V (iirc) so it was a
bit higher.  I use 5 for a conservative (and easier to
do in my head) number.  That works out to be a bit
less than 400 mW.  

Something that I wanted to look at but never got to
was to run the 595s at a lower Vcc to get output high
to be the LED voltage drop (lets say 2V for arguments
sake).  Then the max dissipation would be 2*8*10
(160mW) and eat less battery.

You will see more voltage drop with a higher current. 
It wasn't too bad at 10mA as I recall.  I still don't
think drawing < 1/3 the max spec is abusive and my
experience reinforces that.  

Phil

--- Bob Ammerman <rammerman@verizon.net> wrote:
> 
> ----- Original Message ----- 
> From: "phil B" <philpiclist@yahoo.com>
> To: "Microcontroller discussion list - Public."
> <piclist@mit.edu>
> Sent: Friday, May 27, 2005 2:54 AM
> Subject: Re: [PIC] Best Pic for the job
> 
> 
> > The Fairchild 74HC595 datasheet specs a max
> current of
> > 35mA per pin though I wouldn't get anywhere near
> that
> > with out a pretty gentle duty cycle.  The dip
> package
> > has a 600 mW limit, and the SO's is 500 mW.
> >
> > I've run a number of tests on it because I was
> > concerned.  With all 8 outputs constantly driving
> LEDS
> > at 10 mA for 24 hrs there was very little heat
> > increase (a couple degrees F) so the 500 mW limit
> may
> > be conservative.  This was with both Fairchild and
> ST
> > parts.  Any kind of <100% duty cycle is going to
> help
> > here as well.
> >
> > One thing to be carefull of is inrush current and
> > spikes on the clock and serial in pin.  I use a 1K
> > limiter and 100K to ground on both RCLK and SERIN.
> >
> > Phil
> 
> At 10ma the Vol (low output voltage) of the 74HC is
> probably quite a bit 
> less than 1 volt. So, assuming worst case:
> 
> 1 V * 10 mA * 8 = 8mW
> 
> Not much dissipation at all.
> 
> Bob Ammerman
> RAm Systems
> 
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