Bypass capacitor's purpose

Those on-board ICs with high-speed demand sharp spikes of transient current that the power supply cannot deliver. Power supplies are designed to supply a level amount of power across the entire board, rather than short bursts. To solve this issue, a bypass capacitor can be placed near the IC to supply the required current for these quick spikes. A bypass capacitor does this by storing power and then discharging it to the IC when it requires excess current. This gives the power supply time to respond. Following the spike, the bypass capacitor is recharged and ready for the next cycle.

Bypass capacitors are also important for reducing the ground bounce that comes from digital devices that have faster switching times. Bypass capacitors are also used to filter the low-frequency noise caused by the power supply and are helpful with other signal integrity and EMI issues as well.

How many bypass capacitor needed?

The number of bypass capacitors needed for design will depend in relation to the parts that they are assigned to, and how many of those parts are being used. Bulk capacitors in the 10uF range are usually for each voltage drop on the board. They should be positioned where the voltage is developed or where it enters the board. On some devices, they are used in conjunction with high-speed bypass capacitors.

In general, at least one high-speed bypass capacitors in the 0.1uF range should be placed by each IC. They should be placed as close as possible to their respective IC to supply current immediately. I recommend that devices with multiple power pins should have at least one bypass capacitor for each power pin. Although this will use up more board space, it can significantly help to reduce ground bounce.

The high-speed bypass capacitor can be underneath the device on the opposite side of the board, or just off the pins that the bypass capacitor is connected to on the same side of the board.

For circuits requiring multiple bypass capacitors placed near the power pin of a specific device, the capacitors should be placed next to that pin in ascending order of value. For instance, if both a 0.01uF and a 10uF capacitor are specified for a specific device, place the 0.01uF closest to the device with the 10uF outboard of that. In this way, the larger bulk capacitor will recharge the high-frequency capacitor that is closest to the device pin.

When routing a bypass capacitor, you should start from the power or ground pin of the device and go directly to the capacitor pin. From there the route can continue on a via connecting it to the power or ground plane. You should also use a short and wide traces as possible to connect the bypass capacitor, and use as many vias to connect to the power or ground plane as possible.