Low ripple and low noise normally indicate two different features of power supply. Ripple refers to the output voltage variation with switching frequency, which is measured by scope and reduced by using second stage LC filter. Noise normally refers to the voltage variation in frequency range of 100Hz-100kHz, which is usually measured with noise spectrum and limited by unique IC design. For some application which only requires low ripple but not low noise, a second stage LC filter design method is proposed for general purpose peak current mode buck regulators.

Figure 2-2 shows the scheme of buck converter with second stage filter. A second-order low pass filter is formed by inductor L2 and capacitor C2. A new pair of conjugate poles is introduced with the filter, which can reduce the output voltage ripple and noise at switching frequency through the high frequency gain attenuation. The selection method of inductor L2 and capacitor C2 are analyzed in the application note.

Figure 2-3 to Figure 2-5 show the schemes of power designs with second stage filter, correspond to the power designs with first stage sense, second stage sense and hybrid sense. The following summarizes advantages and disadvantages of each design.

• With first stage sense, the feedback sense point is Vo1 and the voltage drop on DCR of L2 can’t be compensated, so load regulation performance is worse. But the stability is good as the double poles of second stage filter are not included in the control loop.
• With second stage sense, the voltage drop on DCR of L2 can be compensated. But the double poles of second stage filter can have obvious impacts on the loop response. When the values of L2 and C2 become larger, the double poles frequency of second stage filter reduces and can be closer to bandwidth, which can cause less phase margin and possible instability. That limits the maximum value of second stage filter components selection and the ability to reduce output ripple.
• With hybrid sense, the feed forward capacitor Cff is connected with Vo1 and upper feedback resistor R1 is connected with Vo2. The AC disturbance of Vo1 can be coupled to VFB and reduce the portion of Vo2 AC disturbance in total feedback. That can help to reduce the effects of second stage filter on loop stability. And the load regulation performance is also good since the DC regulation is based on feedback from Vo2. So the loop stability and output accuracy can be made sure of at the same time with the hybrid feedback sense.

Due to the obvious advantages of hybrid sense, the reference design is based on this sense approach.

Figure 2-6 shows the TIDA-050073 Block Diagram, using a PCM buck converter TPS62933F with second stage filter and hybrid sense. The application note discusses how to analyze hybrid sense loop stability.