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Enhancing RF system efficiency with bypass mode in hysteretic step-down converters Focus: Hysteretic stepdown converters are used to power RF power amplifiers in 2G, 3G and 4 G wireless handsets. Rather than powering the PA directly off the battery, the converter allows dynamic adjustment of the supply voltage and bias current for greater PA efficiency and longer battery runtime. These benefits come at the expense of voltage headroom. This article explains how this tradeoff can be minimized with the use of a bypass FET or a bypass LDO integrated in the hysteretic regulator IC as exemplified by the MAX8989, MAX8805, and MAX8951. The operation of these regulators is explained and the performance of the bypass LDO is compared with that of the bypass FET. As discussed here, the LDO offers benefits such as a smoother transition, increased current-output capability, reduced inductor requirements, and faster transient response.
What you’ll learn: - How to understand the benefits of a hysteric regulator with integrated bypass FET or bypass LDO in wireless handset applications
- How to reduce the dropout voltage of a stepdown hysteretic regulator
Notes: This material was also presented in Maxim’s application note 5599, which is available at http://www.maximintegrated.com/app-notes/index.mvp/id/5599. The images are easier to read in this application note.
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Author & Publication: Nancy Xiong, Maxim Integrated Products, Power Systems Design Europe, Oct 04 2013
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