Electronic – Decoupling capacitors: what size and how many


Lots of chips nowadays require smoothing capacitors between VCC and GND for proper function. Given that my projects run at all sorts of different voltage and current levels, I was wondering if anyone had any rules of thumb for a) how many and b) what size capacitors should be used to ensure that power supply ripple doesn't affect my circuits?

Best Answer

You need to add a couple of more questions -- (c) what dielectric should I use and (d) where do I place the capacitor in my layout.

The amount and size varies by application. For power supply components the ESR (effective series resistance) is a critical component. For example the MC33269 LDO datasheet lists an ESR recommendation of 0.2Ohms to 10Ohms. There is a minimum amount of ESR required for stability.

For most logic ICs and op-amps I use a 0.1uF ceramic capacitor. I place the capacitor very close to the IC so that there is very short path from the capacitor leads to the ground. I use extensive ground and power planes to provide low impedance paths.

For power supply and high current components each application is different. I follow the manufacturer recommendations and place the capacitors very close to the IC.

For bulk filtering of power inputs coming into the board I will typically use a 10uF ceramic X7R capacitor. Again this varies with application.

Unless there is an minimum ESR requirement for stability or I need very large values of capacitance I will use either X7R or X5R dielectrics. Capacitance varies with voltage and temperature. Currently it is not difficult to get affordable 10uF ceramic capacitors. You do not need to over specify the voltage rating on ceramic capacitors. At the rated voltage the capacitance is within the tolerance range. Unless you increase the voltage above the dielectric breakdown you are only losing capacitance. Typically the dielectric strength is 2 to 3 times the rated voltage.

There is a very good application note about grounding and decoupling by Paul Brokaw called "An IC Amplifier User's Guide to Decoupling, Grounding,. and Making Things Go Right for a Change".