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There is a board diagram used as a chart to determine whether the negative feedback will operate in a stable manner. The horizontal axis of the graph is the frequency and the vertical axis is the gain and phase. As the phase lag due to the inductor (L) and capacitor (C) approaches 180°, it creates positive feedback and the output becomes unstable. However, setting the gain to 1 or less (0 dB or less) even when the phase lag is 180° converges the signal and can prevent the oscillations. Connecting a capacitor and resistor near the error amp to reduce the phase lag and adjust to cancel it. This is called “phase compensation”. Previous designs which used an aluminum electrolytic capacitor with a high ESR for the output capacitor did not have this problem. However, the MLCC has insufficient compensation, which causes anomalous oscillations, so caution is required when replacing capacitors. TDK has provided an ESR control MLCC which allows for the design of an arbitrary ESR value. This can be effectively used for phase compensation.

PTM Published on: 2015-09-10