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Step-down converters have basic blocks that contribute to the closed loop system. Figure 2-1 shows a closed loop system for a generic step-down converter.
is the open loop output impedance of the power stage loop gain and is defined as the product of all gains around the loop. When the loop is open, the power stage is running at a constant operating point meaning the controller signal stays constant.
When the loop is closed, converter is running in normal operation. The closed loop output impedance is :
From this equation we can deduct that the higher the loop gain is, the more the output impedance is reduced by the feedback loop. While designing a power supply, reducing the output impedance is possible by adding output capacitance to lower the open loop impedance. Especially for high-current operation, making sure that the output impedance is low across the frequencies is important to enable good load transients. Though, this method comes to a size and bill of material cost, it might also degrade performance for some designs where a maximum output capacitance should not be exceeded.
From the previous formula we can derive the loop gain: