Predicting load transient response of output voltage in DC-DC converters

In this paper, a method is presented to estimate the amount of over/undershoot and the settling time of the voltage response of a DC-DC converter due to load transients. The method uses the phase margin and crossover frequency of the voltage loop frequency response and the amount of load current change DI to estimate the peak voltage deviation during a load transient. The phase margin and loop crossover frequency is calculated using a unique macro model of the converter which is based on measurement, rather than the usual linearized analytical model and is accurate within a few percentage points for both the phase margin and crossover frequency. The method uses the fact that the behavior of converter+load can be approximated by a second order RLC circuit and maps the dynamics of the latter to estimate the dynamic response of the converter+load system. The predicted and measured response of a 40A, 1.2V DC-DC converter is presented to show the close agreement between estimated and measured voltage deviation. The method can be used for determining the amount of capacitance that should be added to satisfy the output voltage undershoot/overshoot requirements during transients and hence be used to test what-if scenarios before committing to hardware.