58.2.1 Decoupling Capacitors

The use of decoupling capacitors on power supply pins, such as VDD and AVDD, is required.

Consider the following criteria when using decoupling capacitors:

  • Bulk capacitors: Each primary power supply group VDD, AVDD, VDDCORE, and VBAT should have one bulk capacitor. The use of a bulk capacitor is recommended to improve power supply stability. Typical values range from 4.7 μF to 22 μF ceramic or tantalum capacitors with low ESR. This capacitor must be located as close to the device as possible.
  • Value and type of capacitor: All power pins must have a 100 nF bypass cap. A value of 0.1 μF (100 nF), 10-20V is recommended. The capacitor should be a low Equivalent Series Resistance (low-ESR) capacitor and have resonance frequency in the range of 20 MHz and higher. It is further recommended that ceramic capacitors be used.
  • Placement on the printed circuit board: The decoupling capacitors should be placed as close to the pins as possible. It is recommended that the capacitors be placed on the same side of the board as the device. If space is constricted, the capacitor can be placed on another layer on the PCB using a via; however, ensure that the trace length from the pin to the capacitor is within one-quarter inch (6 mm) in length.
  • Handling high-frequency noise: If the board is experiencing high-frequency noise, upward of tens of MHz, add a second ceramic-type capacitor in parallel to the above described decoupling capacitor. The value of the second capacitor can be in the range of 0.01 μF to 0.001 μF. Place this second capacitor next to the primary decoupling capacitor. In high-speed circuit designs, consider implementing a decade pair of capacitances as close to the power and ground pins as possible, for example, 0.1 μF in parallel with 0.001 μF.
  • Maximizing performance: On the board layout from the power supply circuit, run the power and return traces to the decoupling capacitors first, and then to the device pins. This ensures that the decoupling capacitors are first in the power chain. Equally important is to keep the trace length between the capacitor and the power pins to a minimum thereby reducing PCB track inductance.