TABLE OF CONTENTS Ultimately, the power supply is only part of a larger system. Therefore, besides being concerned about the effect of noise and ripple on the converter itself, we need to worry about its effect on the rest of the system. The good news is that if the system were excessively noise sensitive, no one would have touched switchers with a ten-foot pole (or a 10dB zero) in the first place. They would have been using those low-noise, power-guzzling LDOs (linear regulators) instead!
When a customer comes back and complains of too much output noise and ripple on his or her switching converter, pay credence to the ripple if you may, but the noise could simply be an artifact of his or her measurement technique (or lack thereof). Do ask the question! You may be surprised to find that a fairly large percentage have measured the ripple as in Figure 3-1. Or worse, as shown later in Figure 3-3! Notice that a large radio-receptor pickup coil has effectively been created by the scope ground lead in the first figure, and a huge radiating antenna of circulating current in the second (if a voltage differential exists along the ground plane, it will certainly drive a circulating current here). If you don't believe me, connect your probe as shown in Figure 3-2. No, this figure is not a misprint. It is slightly different from the preceding figure! You will be surprised to see what all you are...
