You’ve taken great care in building your design, and you can’t wait to try it out and see the results. Unfortunately, when the time comes to turn it on, you’re disappointed; it’s not working quite as well as it should be, and the lack of efficiency in the transitions when your part is demanding power is particularly troubling, as it leads to chip brownouts or resets.
If you’ve double checked your build but you’re still battling against reactance, you may want to look into adding bypass capacitance.
Bypass capacitance is often a crucial element in making a design workable. Decoupling prevents unwanted interaction between power supplies by relying on the use of bypass capacitors as power storage devices, thereby eliminating inefficient transitions that might take place without the use of these capacitors. As such, it’s a practice you don’t want to leave out of your design. Let’s examine this concept a little further.
Without bypassing, you’ll run into problems with inductance. When current demand changes, inductance causes the voltage to drop. In consequence, the main power supply won’t be able to provide power fast enough, causing the voltage to sag. Then, when the load shuts off, the voltage overshoots. In other words, the flow of current isn’t instantaneous. Unfortunately, to have nice smooth transitions, you really need an immediate supply of current, and you need a place to store excess energy when the transition takes place the other way around. That’s where decoupling comes in.
To address this issue, you need to use a bypass capacitor, which will serve as a source of energy when a switch takes place. When current demand changes, the device will initially draw power from the capacitor, and its immediate need is quickly met. Bypass capacitors therefore give current from the main supply time to catch up when a switch occurs. Conversely, they will also pick up any excess energy once the load shuts off, which prevents a surge in energy (and another bumpy transition) from taking place.
When selecting and incorporating bypass capacitors, keep a few things in mind. First, you need to be sure to use a capacitor that is large enough to meet a sudden demand in your load and to receive excess power once the load shuts off. Additionally, it’s a good idea to place the capacitors as close to the chip’s power leads as possible to ensure your transitions are as efficient as they can possibly be. Also note that while high frequency designs are more susceptible to failure with regards to inductance (simply because there are many more transitions in these designs than there are in low frequency designs), you should also take care to incorporate bypass capacitance in your low frequency builds as well.
Bypassing is an integral part of designing a high functioning build. Hopefully we’ve shed a little more light on the purpose and function of bypass capacitors so you can implement them into your builds and get the stable results you’re looking for. Happy designing!