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Purpose of Surface Mounted Capacitors on CPU

Victor Mehta
Intellectual 275 points


Hello this question may not apply to the TM4V processor but could someone please explain what the purpose of a SMC located directly on top of a CPU ?

Does these types capacitor directly feed the internal circuitry of the CPU with a charge ? Perhaps directly to a specific layer on die of the CPU ?

  • 0
    Guru 10570 points

    Victor Mehta said:
    Hello this question may not apply to the TM4V processor but could someone please explain what the purpose of a SMC located directly on top of a CPU ?

    I've not seen caps mounted on top of a cpu unless there was a mistake on the PCB.

    Perhaps you mean on the bottom of the PCB? Or next to the CPU?

    In either case they are there to provide short term charge storage, compensating for the inductance of the power traces.

    Classically called de-spiking caps because they reduce the size of the spikes on the power supply pins.

    Usually two or three values so that a range of frequencies is covered, smaller capacitors are capable of higher frequencies (lower lead inductance) but have less charge storage. The power planes themselves provide a low value, high frequency capacitor.

    Typically you would also have bulk capacitance distributed around for lower frequency demand , further decoupling the local supply from power lead/plane inductance.

    Robert

  • 0 in reply to Robert Adsett72
    Intellectual 275 points

    Hello and yes my question does not exactly apply to the TM4V processor but if you look at the photos there does seem to be a few of these caps located right on the top of the CPU.

  • 0 in reply to Victor Mehta
    TI__Guru**** 244400 points
    Hello Victor

    While questions are welcome, I would suggest moving non-TM4C questions to stackoverflow.
  • 0 in reply to Victor Mehta
    Guru 27665 points
    As Amit suggested StackOverflow might be a better place (or comp.arch.embedded) to ask.

    A couple of notes. These are not microcontrollers but rather microprocessor assemblies. In fact I rather suspect they may be multi-chip assemblies. In the top one you can see the traces of a PCB. AIUI that PCB serves to bring out the chips I/O to the PGA on the bottom of the assembly. Putting de-spiking caps there would avoid the extra lead inductance of the packaging and socket. At the speed these are likely to be operating at that inductance could well be significant.

    Robert