From: "Tom Faloon"
Subject: Re: power consumption
Date: Fri, 11 Oct 2002 19:36:57 +0100
NNTP-Posting-Date: Fri, 11 Oct 2002 18:32:07 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
In most modern logic gates there are two elements contributing to power
Total power = DC power dissipation + AC power dissipation.
The DC power dissipation is the amount of power dissipated if the inputs and
outputs are static, in defined logic states.
It is calculated from Supply Voltage * quiescent current. In modern 'xMOS'
devices the quiescent current is usually very small. In the case of the
74AC00, it is 20uA max. So maximum DC power dissipation @ 5V supply,
Pdiss-dc = 100uW
The AC power dissipation is a result of switching.
If there is any change of logic states, internal switching transistors have
to charge or discharge stray capacitance within the device, and this draws
extra current from the supply. If they are switched repeatedly can result in
appreciable current being drawn from the supply. The faster the rate of
switching, the greater the current.
If one element of a 74AC00 is clocked, the AC power dissipation can be
Pdiss-ac = Cpd * V(squared) * frequency. (See Fairchild application note
AN-303 for derivation of this formula.)
Where Cpd is the manufacturers estimate of the effective 'switching
capacitance' , V is the supply voltage, and f is the frequency in Hertz.
The Fairchild data sheet specifies Cpd as 20pF, so if you run it at 10MHz,
with a 5V supply, you can expect the AC power dissipation, for one element,
to be Pdiss-ac = 20E-12 * 5^2 * 10E6 = 5mW.
So if all four elements were driven at 10MHz, the total dissipation
Ptotal = Pdiss-dc + 4 * Pdiss-ac = 100uW + 4 * 5mW = 20.1mW
This is much less than the value which you calculated. Perhaps you can see
where you (or I ?) went wrong.
You would have to run all four elements of the 74AC00 at 100 MHz to achieve
200mW dissipation, but this is way beyond it's guaranteed operating speed,
so it won't happen.)
Of course many complex chips, dissipate much more than 200mW !
You will find a useful application note 'AN-303 HC-MOS power dissipation'
and a Fairchild data sheet for the 74AC00 at
Peter wrote in message
> 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.