August 2007

Rev 1

1/16

AN2389

Application note

An MCU-based low cost non-inverting

buck-boost converter for battery chargers

Introduction

As the demand for rechargeable batteries increases, so does the demand for battery

chargers. There are different kinds of design solutions available for implementing battery

chargers. Some of them are dedicated hardware based solutions and some are

microcontroller based solutions.

In a microcontroller based solution, you have the flexibility of using the same hardware for

charging different batteries and making only slight changes in the software. But there are

still some challenges and one of the major challenges is to have a suitable input power

supply available. Generally the Buck converter topology is used as a DC- DC converter to

provide the controlled output power supply to the batteries. But in this case a problem may

arise, for example, if you want to charge a 4.2V Li-ion batteries from a 5V supply due to the

presence of the protection diode and other small drops across other components. This drop

is generally about 1V which makes it very difficult to provide 4.2V to the Li-ion batteries

using the buck converter topology.

This application note describes a simple technique for implementing a non-inverting buck-

boost converter which requires only one inductor. This converter is basically the result of

cascading a Buck converter with a Boost converter. This converter can be controlled by two

PWM signals from the microcontroller and can be used as a Buck converter or Boost

converter whenever required. So this solution combined with the flexibility of the ST7

microcontroller can be used to charge a wide range of the batteries using the same

hardware.

The example used in this application note is specific to battery chargers but this DC-DC

converter can be very useful for portable applications in general or any application which

uses rechargeable batteries.

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