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Frequency-power relationship of circuits

Discussion in 'Electronic Basics' started by Ben Weaver, Sep 25, 2003.

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  1. Ben Weaver

    Ben Weaver Guest

    Hi all...
    Can someone help me? Physicsy question I'm not too sure about:

    If you've got a circuit, for example a microprocessor, running at one
    clock frequency and all of a sudden you double the clock frequency -
    you'd expect the power consumption of the device to go up.

    Question one (reality check): Is this indeed the case?

    Question two: How much would you expect the power consumption to go up?
    Double? Is there a simple relationship? Is it linear?

    Question three: It has been suggested to me that this only applies for
    CMOS devices and bipolar devices have no frequency-power dependance. Any

    Can anyone offer a quick explanation to set me straight on this?


  2. Roy McCammon

    Roy McCammon Guest

    approximately yes. You can think of each clock transition
    as dissipating a fixed amount of energy.
    Bipolar also has a frequency dependence, but it also
    has a large zero frequency bias. Mathematically
    you might call it affine (linear plus a constant).
  3. Yes. CMOS dynamic power is proportional to the clock frequency.
    First, there are two components of power. Dynamic power (switching
    power) is dissipated by changes in state. Dynamic power is goes pretty
    much along the classical P~FV^2 relationship. The more changes in state
    per unit time (Frequency) the higher the power. Static power is the
    power dissipated with no state changes and is proportional to V^2
    (sorta, see below).

    Older true CMOS processes have very little static power dissipation so
    the power dissipated in an older processor goes pretty much along the
    FV^2 dynamic power formula. However, newer processes have a lot of
    leakage, so static power becomes significant. Thus the linear
    relationship doesn't hold (unless you subtract off the static power).
    Certainly there is a frequency/power relationship in bipolar devices as
    well. The difference is that a bipolar device has a far higher static
    power so it'll tend to mask the change in the dynamic power. Advanced
    CMOS processes have high static power consumption (leakage is getting
    to be a big problem) so the relationship is getting more complicated.
    I hope that was quick enough. ;-)
  4. Ben Weaver

    Ben Weaver Guest

    That's great. Very interesting.

    Thanks Keith and Roy.

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