11-05-2012, 02:02 PM
power moisereducton
powernoisereduction.pdf (Size: 364.49 KB / Downloads: 44)
INTRODUCTION
Many of the problems that appear out of Murphy’s box upon transforming a design from the
mythical world of textbooks and SPICE to the real world emanate from the non-ideal power
supply. Real power supplies can cause noise and spurious oscillations that can force the designer
into a frustrating glitch hunt.
Rules of thumb can usually be applied successfully to simple problems, but a little understanding
and forethought will usually provide clean solutions to even the more obscure problems. With
this paper, I hope to provide the understanding of some of the dynamics of power distribution.
The forethought is up to you.
DEFINITIONS
Bypassing and decoupling are often poorly understood and poorly applied. Many designers
believe bypassing and decoupling are synonymous. They are not; they are distinct concepts and
each is a solution to a different problem (see Figure 1).
Bypassing is the reduction of high frequency current flow in a high impedance path by shunting
that path with a bypass, usually a capacitor (in this case, Crypt). Bypassing is used to reduce the
noise current on power supply lines.
BYPASSING
Due to the finite bandwidth of all voltage regulators, their output impedance increases with
frequency. This can be modeled as an inductor in series with the output. Typical values lie
between 1