Digital System Board should have enough decoupling capacitors. Ideally each VCC pin of a device should have a decoupling capacitor. If the device has VCC pins pretty close to each other (like pins 1,2,3 & 4) then upto 4 VCC pins can be combined and provided with one bigger decoupling capacitor. Also refer to manufacturers recommendations for special devices like processors etc. Processors exiting sleep mode require a large amount of current quickly and the power supply and the associated decoupling needs to be designed to support such changes in current requirement. If the board is operating at a frequency more than 100 MHz, then it is recommended to use low ESR MultiCap (say 2.2 uF) as decoupling capacitors. If sufficient 2.2uF MultiCap are used on the board for decoupling, then the second stage of decoupling through bulk tantalum capacitors may be avoided.

Calculation of Decoupling Capacitors

The amount of current needed for switching an output from low to high(I), the number of outputs switching(N), the time required for the capacitor to charge the supply line (dt), and the drop in the supply voltage that can be tolerated (dV), must be known for computation of decoupling capacitor value. C = [I x N x dt] / dV.

For example, consider a buffer with 8 switching outputs. With dV = 0.1V, dt = 3ns, N=8 and I = 44 mA,
C = [44 mA x 8 x 3ns] / 0.1V –> C = 0.01056 uF.
Note: The supply current (I) that is required while the output is switching, can be determined by frequency vs Icc plots for the device, or from the rise time characteristics for typical load. So the decoupling capacitor chosen in this case should be greater than the calculated value.

Few Recommendations for Decoupling Capacitors in Board Design

  • Distribute some bulk tantalum capacitors across the board between VCC and GND pins. The capacitor value may be in the range of 10 uF to 22 uF. On an average, for every ten small decoupling capacitors there should be one tantalum bulk capacitors. If for example the board has 100 small decoupling capacitors then there should be approximately 10 bulk tantalum capacitors need to be distributed across the board between VCC and GND.
  • Use the same value capacitor for decoupling of all the devices on the board. Use only one capacitor value for decoupling of all devices on the board. This value should meet the worst case decoupling requirements for any device on the board. Decoupling would be better if a single valued capacitors are used. Moreover, sourcing of a single valued component is easier than sourcing components of different values.
  • Use low ESR capacitors for IC decoupling. These are better decoupling agents, because of its low ESR value. The equivalent series resistance (ESR) of a capacitor is the resistance of the capacitor leads in series with the equivalent resistance of the capacitor plates. ESR causes the capacitor to dissipate power (and hence produce loss) Low ESR capacitors have higher life cycle and higher ripple current ratings.