SLVAF30 February   2021 TPS55288 , TPS61021A , TPS61022 , TPS61023 , TPS61088 , TPS61089 , TPS61178 , TPS61230A , TPS61235P , TPS61288 , TPS61378-Q1

 

  1.   Trademarks
  2. 1Introduction
  3. 2Observation in Bench Test
  4. 3Root Cause Analysis
  5. 4A Simple Solution
  6. 5Summary

Introduction

A boost converter circuit such as TPS61022 is shown in Figure 1-1. With alternate turning on and off of the two integrated MOSFET, the inductor stores energy and then released to VOUT which is higher than the input voltage. The average value of VOUT is set by the FB pin and R1, R2 resistors.

GUID-20210118-CA0I-PBG2-JXJB-Z8LR2P9KFKT9-low.svg Figure 1-1 Simplified Schematic of TPS61022

The ideal operating waveform at heavy loading condition are shown in Figure 1-2. In the image:

  • IL is the inductor current.
  • IOUT is the output current of the boost converter.
  • ID is the current through the synchronous rectification MOSFET.
  • SW is voltage waveform in SW pin.
  • ΔVOUT is output voltage ripple.
The output voltage drops when the inductor is storing energy, and increases when the inductor energy is released. This behavior results in output voltage ripple ΔVOUT defined by Equation 1. This formula is commonly found in boost converter data sheet. The ΔVOUT is typically lower than 1% of the average output voltage if the boost converter is properly designed.

GUID-20210118-CA0I-BQLH-XTB0-WMJZPQW2RZ7B-low.svg Figure 1-2 Ideal Operating Waveform of a Boost
Equation 1. GUID-20210208-CA0I-XSR4-9NGZ-ZRJ3HSM0NXKT-low.gif

In this equation:

  • COUT is the effective output capacitance.
  • fSW is the switching frequency of the boost.

However, one may observe much larger output voltage ripple than the calculation result in real circuits. This application details the root cause of the observation and proposes solution to solve the problem.