In this section, we explain wiring for feedback of a signal from the output to the FB pin of the power supply IC.
Feedback Path Wiring
Among signal wiring in general, particular care must be taken in designing wiring for feedback signals. As indicated in the circuit diagram on the left in Figure 7-a, the output voltage is divided by resistors and fed back to the FB pin of the power supply IC--that is, an error amplifier input--via wiring, and the power supply IC stabilizes the output voltage based on this voltage information. What is important about the feedback path is that if some noise or fluctuations other than the actual output voltage were sent to the error amp, not only would accurate output stabilization no longer be possible, but under some conditions oscillation or other unstable operation could occur. Consequently, feedback paths must be laid out such that clean signals can be fed back.
Below, precautionary points regarding wiring are explained. Please refer to the diagram on the right side in Figure 7-a.
Actual wiring is designed paying attention to these points. Figure 7-c is an example of layout in which the feedback path is moved to the bottom layer through a via, while giving a wide berth to switching nodes. As can be seen by examining the board, it is not a simple matter to draw wiring in a manner that completely satisfies the above-described conditions. Although not specifically stated above, the essence of wiring is "as short as possible". Hence in the board below, drawing a needlessly long feedback trace that keeps away from the entire circuit cannot be called wise. In such cases, deploying the feedback signal wiring on the bottom layer, as in the example shown, is one option.
Figure 7-d is an example of unsuitable wiring. The feedback path is laid out parallel to the inductor, and so the magnetic field generated near the inductor induces noise in the feedback path.
In actuality, due to relationships with other components, there are cases in which the ideal layout and wiring cannot be realized. In such cases, it will be necessary to consider what exactly is the essence of ideal wiring, and to find a compromise that strikes the best possible balance between competing factors.
・A feedback signal line from the output should be laid out away from the switching nodes. If the line picks up noise, errors and erroneous operation will result.
・Placing wiring on the bottom layer of the board through a via is another option.