Decoupling

Another post today, you'd think I'd leave this thing alone wouldn't you.

Anyways, I'm currently looking at the power decoupling for this QuadPHY I mentioned earlier today. Decoupling is simply a way of preventing undesired coupling between subsystems via the power supply connections. To put simply, if you have two devices working off the 2.5V power plane, you don't want one to start sucking power, leaving the other device short on current. So to prevent this, we connect localized capacitors close to the power leads of integrated circuits to act as a small localized energy reservoir. This will supply the circuit with current during transient, high current demand periods, preventing voltage being pulled down from momentary current loads.

To construct these wells, we use a number of capacitors in series. A surface mounted, ceramic multi layer chip capacitors (SMT, MLCC) are ideal for this situation. However greater capacitance may be needed depending on the voltages and currents being applied.

The rule of thumb with voltage rating for a capacitor is usually between 1.5 to 2 times the voltage being used (this is specified in the Military Handbook 217F. It's freely available online, but it still sounds cool when you're reading official military documents). So if you have a circuit that draws 3V, then it's best to use a voltage rating of 4.5V. Common rated caps come in 6.3V though, so you'd probably use this one instead. The temperature coefficients are very important when choosing the capacitor as well. X5R and X7R are the better quality caps to go for. They're the best priced too. But I'm not going to go into Temperature coefficients here, thats way complicated and not interesting. You can find out more about it on Wikipedia.