8.2.2.2 Output Filter Selection

The output filter used with the TPS560200-Q1 is an LC circuit. This LC filter has double pole at:

Equation 3.

At low frequencies, the overall loop gain is set by the output set-point resistor divider network and the internal gain of the TPS560200-Q1. The low frequency phase is 180 degrees. At the output filter pole frequency, the gain rolls off at a –40 dB per decade rate and the phase drops rapidly. D-CAP2 introduces a high frequency zero that reduces the gain roll off to –20 dB per decade and increases the phase to 90 degrees one decade above the zero frequency. The inductor and capacitor selected for the output filter must be selected so that the double pole of Equation 3 is located below the high frequency zero but close enough that the phase boost provided by the high frequency zero provides adequate phase margin for a stable circuit. To meet this requirement use the values recommended in Table 2.

Because the DC gain is dependent on the output voltage, the required inductor value increases as the output voltage increases. Additional phase boost can be achieved by adding a feed-forward capacitor (C5) in parallel with R1. The feed-forward capacitor is most effective for output voltages at or above 1.8 V.

The inductor peak-to-peak ripple current, peak current, and RMS current are calculated using Equation 4, Equation 5, and Equation 6. The inductor saturation current rating must be greater than the calculated peak current and the RMS or heating current rating must be greater than the calculated RMS current. Use 600 kHz for f_SW.

Use 600 kHz for f_SW. Make sure the chosen inductor is rated for the peak current of Equation 5 and the RMS current of Equation 6.

Equation 4.

Equation 5.

Equation 6.

For this design example, the calculated peak current is 0.582 A and the calculated RMS current is 0.502 A. The inductor used is a Würth 744777910 with a peak current rating of 2.6 A and an RMS current rating of 2 A.

The capacitor value and ESR determines the amount of output voltage ripple. The TPS560200-Q1 is intended for use with ceramic or other low-ESR capacitors. The recommended values are given in Table 2. Use Equation 7 to determine the required RMS current rating for the output capacitor.

Equation 7.

For this design two MuRata GRM32DR61E106KA12L 10-µF output capacitors are used. The typical ESR is 2 mΩ each. The calculated RMS current is 0.047 A and each output capacitor is rated for 3 A.