AN2389 Datasheet, PDF (1/16 Pages) STMicroelectronics – An MCU-based low cost non-inverting buck-boost converter for battery chargers

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.
August 2007
Rev 1
1/16
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