AN1593 Datasheet, PDF (1/8 Pages) Motorola, Inc – LOW COST 1.0 A CURRENT SOURCE FOR BATTERY CHARGERS

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AN1593
Low Cost 1.0 A Current Source
For Battery Chargers
Prepared by: Ondrej Pauk
Industrial System Application Laboratory
Roznov, CZ
Figure 1. Low Cost Current Source for Battery Chargers Demonstration Board
This paper describes two designs of low cost current
sources for battery charger applications based on the
LM2575–ADJ switching step–down converter and the
MC33341 regulator control circuit.
INTRODUCTION
This design is a highly cost effective 1.0 A current source
for battery chargers with a rectangular constant–current,
constant–voltage charging characteristic. This feature
assures a basic protection against overcharge whose results
can range from minor damage to catastrophic failure of the
whole system.
This circuit was designed to implement additional charge
control based either on the microcontroller or on any other
charging control unit in the system that operates from NiCd or
NiMH batteries. The MC33341 and this board may be used in
a wide variety of applications. All functions needed are
performed by just two integrated circuits and a small number
of external components. This allows a very compact printed
circuit board design and a very cost effective solution.
The LM2575–ADJ Easy Switch™ step down converter
allows the system to operate from 8.0 to 40 Vdc, thus
allowing direct operation from both 12 and 24 V board
voltages used in the automotive industry. In comparison with
linear topologies of battery chargers, this circuit provides
much better efficiency, especially over a wide input voltage
range.
© Motorola, Inc. 1997
General Description
Today’s most popular rechargeable battery type is NiCd.
When overcharged, this type of battery experiences
increasing pressure inside the cell. This can cause opening
of the cell’s vent and release of oxygen. This has a
detrimental affect on the battery, although it may still retain
some useful capacity. When NiMH batteries are
overcharged, they also increase their internal pressure and
release some hydrogen, an extremely explosive gas.
The schematic diagram is shown in Figure 2. It is a 1.0 A
(maximum) “dumb” battery charger that uses the
LM2575–ADJ switching converter to step down the input dc
voltage, together with the MC33341, which regulates the
charging current flowing into the battery. The switching
regulator has high efficiency over a wide input voltage range,
which allows this design to be universal. Both 12 and 24 V car
batteries as well as cheap, poorly regulated, wall adaptors
can be used.
The term “dumb” battery charger means that it offers only
some basic protective features and the main protective and
control functions are maintained by a µP based main control
unit inside the PC or the control function of a cellular phone.
This concept allows a very compact and cost effective
design.
Various charge techniques have to be used to
accommodate both NiCd and NiMH type batteries. Both NiCd
and NiMH batteries can be charged at a high current rate