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/ dana@locus.com (Dana H. Myers) /  2:14 pm  Jan 10, 1991 /

$   The 25Mhz chips DO run hotter than the the 16Mhz parts. CMOS power
$ consumption occurs during transitions. There are two reasons why;(1)
$ current must flow through the gate capacitances (standard 1/(2*pi*f*C)
$ stuff) and (2) there is a brief instant when both the P and N channel
$ FETs are conducting. Resultantly, the power consumption of CMOS parts
$ is roughly proportional to frequency of operation. A 25Mhz will consume
$ about 25/16 (or 150%) of the power a 16Mhz part consumes.
$ 
If I remember correctly, the energy dissipated in a CMOS chip is
proportional to the square of the frequency. So, 25 MHz vs. 16 MHz power
supply would be 25^2 / 16^2 = 244 %. Quite a hell lot more power.
Assuming voltage and impedance stay about constant, heating rate of
the chip will be about proportional to power dissipation. But since
higher temperature will give rise to higher rate of heat loss; may be
a part running at 25 MHz will be about twice as hot as a part running
at 16 MHz.

(Somehow, can't quite remember the equation that says that power is
proportional to freq^2).


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