Why Do I Use So many Bypass Capacitors?

When you look at the design requirements of most ICs they commonly require a .1uF bypass capacitor, with one larger bypass capacitor (on the order of a few uFs) for every 3 or 5 ICs. I use a .1uF and a 2.2uF capacitor for every IC. That seems a bit extravagant both in capacitor costs and board area. So why do I do it?

Let us think about it. An IC draws a pulse of current due to a change of state. This pulse causes a change in voltage on the .1uF bypass capacitor. By having a 20 times larger capacitor right next to the .1uF the change in voltage will be 20 times smaller (actually a bit less due to the frequency response of the larger capacitor). This effectively reduces power supply noise for the IC by a factor of 20. It also decreases the peak of the current pulse needed to recharge the capacitor (the recharge will also take longer). Lowering the peak current also reduces emitted EMI.

If I was going into volume production and had a good EMI lab I could reduce the number of 2.2uF capacitors until it started making a measurable difference. Since I don't have an EMI lab in my shop - I just keep those extra capacitors and come in with a lower noise design. This is especially necessary for high frequency noise since most chips are not very good at reducing that kind of noise.

So why not throw out the .1uF capacitors and just use the 2.2uF caps? Because capacitors are to some extent frequency sensitive. The .1uF caps will respond to fast transients that will not (immediately) affect the 2.2uF capacitors.

Engineering is the art of making what you want from what you can get at a profit.