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Bypass capacitors and bulk capacitors

  1. Apr 12, 2012 #1
    I'm trying to understand the role of bypass caps and bulk caps, applied specifically to FPGA and other digital ckts.
    Bypass caps provide a low impedance path to the noise on the supply lines and also supply switching currents for the logic devices. But what's the role of bulk caps?
    Henry ott's book says, the bypass caps need to charge quickly to make up for the lost charge. The bulk caps charge them up quickly.
    But why can't the supply charge these caps?
    Bypass caps are same as decoupling caps, right?
  2. jcsd
  3. Apr 12, 2012 #2
    Yes bypass caps are the same as decoupling caps. They're used a lot in all electronics, but they're important for FPGAs and other digital devices for a number of reasons.

    First, make sure you understand one of the needs for the caps. The voltage regulator or other power supply has a maximum current output, and when digital circuits draw current, they often do it in large bursts or pulses, and sometimes this exceeds how much the voltage supply can provide instantaneously. If this condition occurs, the voltage level can sag because its being loaded down, and it can even become unstable and fall out of regulation.

    Bulk caps are like big resevoirs of energy, and, ideally, they can supply all of this energy that your digital circuits pull instantaneously. You might want to think of them as a middle man between your power supply and digital circuits to supply the energy, and its not unusual to think of bulk capacitors and bypass caps as both filling the same role of supplying energy very quickly to the circuits that need it.

    Large value capacitors, and numerous of them in parallel, are usually placed on the output of voltage regulators, and I think of these as the bulk capacitors. I think of bypass capacitors as the smaller ones you place right next to the voltage pins of your digital circuits, and their values are dependent on what the part's datasheet recommends or how much current you expect to draw from them.

    The other related purpose of bypass caps is for noise immunity. Like you mentioned, they provide a low impedance path to ground. If you have noise riding on your voltage supply, you can damage your digital circuits, the noise can couple to other parts of the digital circuits, and you can generate errors or cause failure in the chip's normal operation. The capacitors shunt this noise to ground. Another point for this is that the capacitor values must be chosen to shunt the right frequencies of your noise. Bigger capacitors are used for lower ranges, and small caps in the pF or nF range are used for higher frequencies. Its good to use a variety of values in parralel to give a good response to a broad range of noise, and low ESR caps is also important to keep the impedance down. By placing them in parallel, you also are reducing the ESR. Also, you want to place these caps as close to the parts they're used on so that the inductances of the traces does not impede the capacitor's ability to supply charge to the pins.

    One last thing, the bypass caps aren't necessarily just used on the voltage supply nodes. They can also be placed on reference voltages, digital I/O, and pretty much any other signal that can benefit from the bypass, assuming the capacitor does not negatively affect those signals. They can be used for other protection purposes such as ESD damage, usually in conjunction with a resistor, diode, or zener diode, where they are able to grab up all of the charge from an ESD zap before it can build up on the digital pins and blow up the chip's internal components.
    Last edited: Apr 12, 2012
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