1 Introduction

For the power of signal processing system design using ADCs, PLLs and RF transceivers, low output voltage ripple is an important factor for power quality evaluation. In some power design designed to achieve low output voltage ripple, normally a buck converter is used for voltage step down as first stage and an LDO is used to filter ripple as second stage. However, the BOM cost, design size and conversion efficiency can cause concern in some compact or cost-effective application.

In recent years, a new low ripple power design attracts more attention by using a secondary stage passive LC filter combined with buck converter. Compared with conventional design with LDO, reduced design size and efficiency improvement can be achieved. See Powering the AFE7920 with the TPS62913 Low-Ripple and Low-Noise Buck Converter application note. But a pair of conjugate poles can be introduced by the added passive filter, which threatens the loop stability.

After adding second stage filter, different feedback sense networks can cause different impacts on loop stability. The schemes of feedback are divided into three types in this Peak Current Mode Converter Secondary Stage Filter Design for Low Ripple Power – Part I: Filter Design for Output Ripple Reduction application note: first stage sense, second stage sense and hybrid sense. In this paper, the advantages and disadvantages of those three types of feedback sense are compared at first. And hybrid sense is selected as the recommended feedback sense network due to the good performance on load regulation and loop stability. On the basis, the loop response of peak current mode buck regulator with hybrid sense is analyzed and the stability design method is proposed. Combined with the second stage filter components selection restriction in Part I, the application design flow of PCM converter based low ripple power design is proposed. At last, the design method proposed in this application note is verified by experiments with TPS62933F, TPS6293x 3.8-V to 30-V, 2-A, 3-A Synchronous Buck Converters in a SOT583 Package. The TPS62933F Low-Output-Ripple Power Supply With Second Stage Filter Reference Design including schematic and PCB layout design based on the proposed design method.

To be noted, the method proposed in this application note can only be used as a reference material. Due to the simplification in deduction and non-designed for factors in reality, the calculated results can have difference with bench test.