Reverse Current Operation of the LM5177 Buck-Boost Converter
Abstract
The LM5177 is a 4-switch buck-boost converter that supports seamless bi-directional operation, designed for applications like critical system power backups and energy storage from renewable sources. This application note provides a comprehensive guide on setting up the bi-directional current feature of the LM5177, offering clear design guidelines to help users get the most out of the device and optimize the performance in various scenarios. In Battery Backup mode, energy storage connected to the Vin of the LM5177 is charged with a constant current, managed by the built-in current limit features of the device and an external circuit using the TLV431 shunt regulator.
The TLV431 is used for limiting the charging voltage and controls the MODE pin with switching the converter between FPWM and PSM modes to maintain the desired storage voltage with minimal fluctuations. Tests using the LM5177 Buck-Boost Controller Evaluation Module show smooth transitions between constant current and constant voltage charging, with only minor ripple due to mode switching delays. The low operating current and reference voltage boost efficiency of the TLV431, and further adjustments to the MODE pin controller can enhance performance even more.
Overall, the capabilities of the LM5177 and the outlined control strategies offer a reliable design for efficient energy storage and management for a wide range of applications.
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1 Introduction
The LM5177 device is a buck-boost converter that also facilitates seamless bi-directional operation. This feature enables usage in various additional domains. For instance, the device can be used as a reliable power backup for critical systems or to store energy generated by renewable sources.
The primary objective of this document is to provide clear guidance on how to setup the operation for the bi-directional current feature of the LM5177 buck-boost converter. Through detailed design guidelines outlined in subsequent sections, users can gain insights into maximizing the device functionality and optimizing the performance in diverse settings.
The backup operation is effective only when the buck-boost controller is in FPWM mode. To activate this FPWM function, the mode pin of the controller must be set high. If the pin is not set high, the converter runs in PSM mode (forward operation). For Battery Backup mode, the backup energy storage is connected to the Vin of the LM5177. When the power backup operation is enabled and the output voltage exceeds the feedback threshold, the converter charges the storage element with a constant input current. This charging current can be controlled using the built-in current limit features of the buck-boost controller, including peak current limit and average current limit. The upper voltage charging limit needs to be controlled by an external circuit.
Figure 1-1 Overview of Bi-directional Current Flow Operation