Some low-noise applications may require the power supply output
ripple voltage to be less than 0.1 percent of the output
voltage. This low ripple requirement easily can translate into
filter attenuations significantly greater than 60 dB, which
cannot be practically met with a single stage. In Power Tip 54 ,
we discussed the design and time domain simulation of such
filters. In this Power Tip, we discuss using P-Spice in closing
the feedback loop around such a filter. The trick to getting a
low noise output is to employ a two-section filter. However,
with the additional components in the filter comes additional
phase shift, which can wreak havoc with a power supply control
loop. In Power Tip 54 , we discussed a strategy to minimize this
phase-shift by damping the power supply filter and by putting
most of the power supply’s capacitance at the output of the two-
section filter. In this Power Tip, we further minimize the
phase-shift in our control loop by employing peak current-mode
control. This allows us to clo