Power Tip 55: Compensate a low-noise power supply having a two-section filter

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 close the feedback loop at a high frequency with adequate phase margin with the two-section filter.

Figure 1 shows the P-SPICE simulation model of the power supply example we are about to consider. This model is based on the TPS54620, a step-down integrated FET, synchronous buck converter that comprises four sub-circuits: power stage and filter, error amplifier, modulator delay, and output divider. The power stage portion of the model takes advantage of the current-mode control of the controller IC. Current-mode control transforms the output inductor into a voltage controlled current source (VCCS) (G4 in Figure 1), feeding the remainder of the output filter and load resistor.