Frequency limit in current digital cmos circuits

[julian403]

Hello all.

Why there is a frecuency limits on cmos digital circuits? For example the microprocessors. I means, more frecuency more heat. Why there is more heat? Its because the Vgs capacitance and the impedeance goes up with more frecuency? due to it drives more voltage drops?

Thanks in advance.
Regards.

 

[frankrose]

Logic gates only dissipate heat when output transition happens. The transition time (duration under a logic gate's output can change) is constant, depends on Cgs and Ron (the process or technology).
Ratio of transition time and clock time period arises when the clock frequency increases. If the ratio is higher, the dissipated power is higher, because the logic gates spend more percentage of a time period in output transition.
The ratio can't be higher than ~0.25, otherwise the gates won't have enough time to change output value. This limits the frequency theoretically, practically they will produce too much heat under this ratio, which would require more cooling, bigger heatsinks, which is not possible in most of the cases, and other delay problems would occur, like race condition between lines.

 

[BradtheRad]

The device takes a certain amount of time as it goes from On-to-Off or Off-to-On. Therefore the output starts to look like a triangle wave the faster you try to make it do transitions.
Besides which, the device generates more heat in the middle region between fully Off or fully On. So it heats up more as you cause it to do more transitions per unit of time.

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Now I see Frankrose replied more quickly than I, along similar lines.