Technical Information Site of Power Supply Design

2021.04.21 DC/DC

Feedback Path Wiring

PCB Layout of a Step-Up DC/DC Converter

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.

  • (a) The FB pin of the IC, to which the feedback signal is input, normally has high impedance; short wiring should be used to connect this FB pin to the resistor divider circuit using R1 and R2.
  • (b) The place at which the output voltage is detected should be either the terminals of the output capacitor COUT, or should be connected behind the output capacitor.
  • (c) Noise tolerance is better if the wiring of the resistor divider circuit is parallel and in close proximity.
  • (d) Wiring should be far away from the inductor L and diode D2. Wiring should not be located directly below the inductor or the diode or parallel to power system wiring. The same is true for multilayer boards.

Key Points:

・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.

PCB Layout and Selection of Peripheral Parts

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