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Bypass Capacitors for PowerPC

The hardware speciﬁcations state that one capacitor should be used per power pin,

and that the values should range from 0.01uF to 1.0uF. Is all that really necessary?

The intention behind the bypassing section is to help insure a good ground return path

for the processor and MPC106 devices, and to insure that there is good transient

response for the power supply for these devices, which are switching rapidly internally

(100-400 MHz). Providing good quality power and ground signals cannot be underesti-

mated.

The actual number of high-frequency (ceramic) bypass capacitors needed need not be as

high as stated, assuming a good multi-layer PCB. Due to the physically small size of the

packages for the MPC106 as well as most newer PowerPC processors, which are all

BGA, installing beyond 10-12 ceramic capacitors surrounding the device (two per side,

possibly two beneath) will produce diminishing returns. With the above number of

capacitors, the ground return path deﬂection is no more than 1 cm away from any pin on

a 255 or 360 pin BGA device.

The value of capacitors needed for bypassing can be also restricted to a single value, typ-

ically 0.1 uF. The need for a range of values was only important when leaded bypass

capacitors were still used; the long pins of those devices caused excessive lead induc-

tance, making them ineffective at resonant frequencies. Newer SMD capacitors, partic-

ularly those in “0805” packages have so little inductance the issue is no longer

important.

The bulk capacitors, those 10uF or higher, have no critical placement and are used to

provide a stable voltage plane for the bypassing capacitors.