Feedback trace

I also like to try to ensure that the feedback trace has a quiet environment—perhaps ensconced between ground traces on either side, but certainly never a switching (high-frequency) trace running alongside. 

We are also seeing another pattern emerge here, that the inductor and its associated traces are not critical in any topology. That is because the inductor smooths out the current through it, so obviously no edges of current pass through it. The slowly undulating inductor current has ripple, but not noise We need not pay very close attention to it, except to keep it away from sensitive nodes, in particular the feedback trace. 

Keeping the feedback trace short may not always be physically feasible. We should realize that keeping it short is certainly not of the highest priority. In fact, we can often deliberately make it long, just so that we can assuredly route it away from potential noise sources. We can also judiciously cut into the quiet ground plane to pass this particular trace through, so that it is, in effect, surrounded by a sea of tranquility. ... 

Recommendation 7 (Figure 6-14) The divider needs to be physically close to the IC to avoid noise pickup along the feedback trace. The feedback trace doesn t run under the inductor or diode in particular, and is in fact kept at least a couple of millimeters away from the body of the diode. Also note that a test point has been created for this node, too. 

Figure 6-14 Voltage Divider Placed Physically Close to the IC to Avoid Noise Pickup Along Feedback Trace... 

But the middle of the divider is still fixed at 1.25 V, since the feedback trace carries almost no current. 

Note that in all topologies, the inductor is not in the critical path. So we need not worry much about its layout, at least not from the point of view of noise. However, we have to be wary of the electromagnetic field the inductor creates, because that can impinge on nearby circuitry and sensitive traces, and cause similar (though usually not so acute) problems. So generally, it is a good idea to try and use shielded inductors for that reason, if cost permits. If not, it should be positioned a little further from the IC, in particular keeping clear of the feedback trace. 

The only important signal trace to consider is usually the feedback trace. If this trace picks up noise (capacitively or inductively), it can lead to slightly offset output voltages — and in extreme cases (though rare), even instability or device failure. We need to keep the feedback trace short if possible so as to minimize pickup and keep it away from noise or field sources (the switch, diode, and inductor). We should never pass this trace under the inductor, or under the switch or diode (even if on opposite... 

---