For D-CAPx regulators, the PWM
modulation gain is determined by the falling slope of the triangular waveform formed at
the FB pin by the DCR injection network and output-capacitor equivalent series
resistance (ESR). The parasitic inductance and resistance along the injection cable and
noise coupled into the wires distorts the triangular waveform at the FB pin, which
renders a different PWM modulation gain than the regulator with an improper test setup.
To preserve accuracy, a bypass capacitor, C
pass, is added in parallel to a
20-Ω resistor by forming a high-pass filter. The corner frequency is set lower than
one-half of the switching frequency of the converter so that the triangular waveform at
the FB pin during the testing remains similar to that during normal operation. A 0.22 µF
capacitor is used for a converter switching at 500 Hz in this example. For most
applications, the proper C
pass value would be from 0.1 µF to 0.47 µF. To
minimize the effect on the system, the DCR injection capacitor, C
p, should be
less than one-tenth of C
pass, as
Figure 1-3 shows.