Feedback Path Wiring

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.

• ・When feedback signal wiring picks up noise, an error occurs in the output voltage, and in some cases operation becomes unstable.
• ・The FB pin of the IC, to which the feedback signal is input, has high impedance, and so this pin should be connected to the voltage division node of a resistive voltage-dividing circuit using wiring that is as short as possible: see (a) in the diagram
• ・The place for detecting the output voltage should be either across the terminals of the output capacitor, or past the output capacitor: see (b) in the diagram
• ・Lines from the output to a resistive voltage divider should be parallel and close to each other so as not to easily pick up noise: see (c) in the diagram
• ・Wiring should be laid out far from the switching nodes of inductors and diodes: see (d) in the diagram
• ・Wiring should not be placed immediately below an inductor or diode, or in parallel with wiring of a power system (even in multilayer boards)

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.