Here, we explain wiring for feedback paths.
Feedback Path Wiring
As was explained in "Current Paths in Step-up DC/DC Converters", in circuit wiring there are paths related to input and output in which large currents flow, and also paths used for control and the like in which only small currents flow. In general, paths in which only small currents flow are signal system paths. In addition to the wiring of the feedback path that feeds back the output voltage to the FB (feedback) pin, there may be, depending on the power supply IC, signal wiring to enable or shutdown pins for on/off control.
Among signal system wiring, the wiring for feedback paths in particular requires special attention. This wiring feeds back the output voltage from the output line to the FB pin in order to regulate the output voltage, and is connected to the input of an error amplifier within the IC via a voltage-dividing resistor used to set the output voltage, and therefore is a high-impedance line. When the line impedance is high, it tends to pick up noise, and if the wiring of a feedback path picks up noise, errors occur in the output voltage, and operation may become unstable.
Matters requiring caution where feedback path wiring is concerned are indicated below. The circuit diagram on the left is of a circuit with simple connections; the circuit diagram on the right reflects places and wiring corresponding to the four matters, (a) through (d), requiring caution.
・Feedback path wiring has high impedance and picks up noise easily.
・When feedback path wiring picks up noise, errors may occur in the output voltage, and operation may become unstable.
・The four matters noted in the text require attention when laying out wiring for feedback paths.