From: John Larkin
Subject: Re: power consumption
Date: Fri, 11 Oct 2002 10:59:01 -0700
Organization: Posted via Supernews, http://www.supernews.com
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On 11 Oct 2002 08:40:13 -0700, firstname.lastname@example.org (Peter) wrote:
>Hi you all!
> i would like to ask how to calculate the power
>consumption of IC's from the info in datasheet provided, say 74ac00. I
>am aware that there are some equations that take in to account the no
>. of input output pins, the freq of these pins working, Vcc, Icc and
>the loading capacitance. However the power calculated from this is
>quite large ,(200mW) and if such IC is operated ,it seems a bit
>unrealistic if it is to be supported using batteries.
>Do forgive me if this seems a stupid question, but help me out by
>pointing out where i am wrong.
I don't know which equations you're using, but 200 mw for an AC
running loaded at high speed is entirely reasonable.
The 'switching' current drawn by each section of a CMOS part is
I = C * V * f
where V is usually 5 volts
f = switching frequency
C = external load capacitance plus the internal 'power dissipation'
capacitance, often shown on datasheets. This represents actual
internal node capacitances plus a fudge factor for shoot-through
current, typically 30 pF total for an AC00 gate. All this assumes fast
At 100 MHz, no external load, this gives about 3 ma, 15 mw per gate.
But at 1 MHz, it's 1/100 as much.