Order this document by AN1593/D
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.
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
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
AN1593
(‘c’ rate) up until the charge limit is reached. After that, the
battery has to be charged by a much lower current at the so
called ‘trickle charge’ rate. Trickle charging is a continuous
low current charging rate that keeps the battery fully charged.
While NiCd batteries have a recommended trickle charge
rate of about c/10, for NiMH type it is not recommended to
exceed a charging rate of c/40.
Some battery manufacturers recommend, for their
chemistry, pulse charging instead of continuous current
charging. This feature can be accomplished by use of the
ON/OFF pin of the LM2575.
In the circuit shown in Figure 2, the MC33341 control
circuit is configured for high–side current sensing. The
voltage drop across the sense resistor RS provides a voltage
that is proportional to the charging current. The current
regulation threshold V
sen
can be adjusted externally (switch
S1 in position “2”) in the range of 0 V to 200 mV with respect
to Pin 4 of U2. When the switch S1 is in position “1”, the
current regulation threshold level is set internally to 200 mV.
Then the regulated current can be calculated as follows:
I reg
sen
0.2
+
VRS
+
RS
Circuit Operation
Circuit operation is as follows. When a discharged battery
is connected to the charger, the circuit operates as a constant
current source. The LM2575–ADJ buck regulator is used to
step down an unregulated dc input voltage. This regulator is
capable of providing up to 1.0 A of charging current.The
amount of charging current flowing into the battery is
controlled by the MC33341 regulation control circuit. This IC
is used to control the feedback loop in either
constant–current or constant–voltage mode with automatic
crossover. The MC33341 features the unique ability to
perform both high–side and low–side current sensing, each
with either internally fixed or externally adjustable threshold
level. This feature makes this circuit very universal and
ideally suited for use in connection with a microcontroller
based intelligent control systems.
Resistor R3 is required in those applications where a high
peak level of reverse current is possible, if the source outputs
are shorted and the diode D2 is not used. The resistor value
should be chosen to limit the input current of the internal VCC
clamp diode to less than 20 mA. Excessively large values for
R3 will degrade the current sensing accuracy. Resistor R3
value can be calculated from the following expression:
R3
+
I
– 0.6
pkRS
0.02
where IpkRS is a peak current flowing through the sense
resistor RS.
Once the battery voltage reaches a predetermined level,
the MC33341 begins to regulate in the constant–voltage
mode and the charger starts to regulate the voltage across
the battery. This voltage is monitored by Pin 5 of U2, the
Figure 2. Low Cost Switching Regulator Performs
Constant–Current/Constant–Voltage 3 Cell Charging Function
ON/OFF
+
+Vin
1
Unregulated
DC – Input
Vin = 10 to 40 V
U1
LM2575–ADJ
5
Output
2
C1 +
100
m
F/50 V
4 Feedback
R4
1.0 k
W
3
Gnd
D1
1N5819
L1
400
m
H
+
C2
330
m
F/16 V
ON/OFF
Control
(from Controller)
Gnd
RS
0.22
W
A
D2
+VO
1N4001
C3
33 nF
8
7
R3
27
W
6
R2
39
W
+
–
3 Battery Cells
Under Charge
1
U2
MC33341
5
R1
10 k
W
–VO
2
3
4
1
2
S1
Charge Current
Control
(from Controller)
2
MOTOROLA ANALOG IC APPLICATIONS INFORMATION
AN1593
non–inverting input of the transconductance amplifier inside
the MC33341. This voltage is divided by resistor divider R1,
R2 to the 1.2 V internally fixed level Vth. By this arrangement
the battery charger output voltage threshold can be set.
Moreover, in the low–side current sensing configuration
(refer to Figure 3) this threshold level can be externally
adjusted over a range of 0 to 1.2 V with respect to the U2
ground at Pin 4. The maximum battery charger output voltage
Vreg (the voltage at the point A with respect to Pin 4 of U2)
can be calculated as follows:
V reg
of a single 1.2 V cell may not be possible (depending on the
voltage drop on D2, if used). Also the current limit in the case
of fully discharged cells or shorted output is given only by the
internal current limiting of the LM2575, as mentioned above.
This drawback can be solved by using the circuit shown in
Figure 3.
Improved Battery Charger
This circuit is very similar to the previous one. It uses the
same source IC, the LM2575 and also the same charger
control IC, the MC33341 but now in the low–side current
sensing configuration.
The second difference is a different connection of the VCC
pin of U2 which is the supply voltage pin of the MC33341.
Now this VCC pin is connected directly to the unregulated dc
input voltage through the supply current limiting resistor R5
and resistor R6. The use of the coupling capacitor C4 is
essential to assure a stable operation of the whole system.
C4 transfers the ac part of the LM2575 output voltage (or the
LM2575 output ripple voltage) through the MC33341 VCC
Pin 7 and output Pin 8 into the feedback Pin 4 of the LM2575.
The way the LM2575 operates makes this connection
necessary. Since the maximum allowable supply voltage of
the MC33341 is 18 V, the Zener diode D3 has to be used to
clamp the supply voltage of the MC33341to its operating limit
when the input voltage exceeds that value. Use of such an
arrangement assures that the charging control circuit U2 will
always have a supply voltage high enough, even under short
circuit conditions at the output of the battery charger. Switch
S1 can be used the same way as in the previous design.
Switch S2 is used to alter the output voltage threshold. When
S2 is in position “1”, the voltage threshold on Pin 5 is set
internally to 1.2 V and consequently the output voltage
threshold can be set only by the resistor divider R1, R2.
Switching S2 to position “2” allows an external control of the
Pin 5 voltage threshold V
th
in the range of 0 V to 1.2 V. This
feature contributes to the universality of this battery charger.
+
Vth
R2
R1
)
1
+
1.2
R2
R1
)
1
The current control loop is closed by connecting Pin 8 of
U2 directly to the feedback input of the LM2575 (Pin 4 of U1).
Under normal working conditions this pin is held at 1.23 V,
resistor R4 is added to convert the MC33341 output current
to this voltage. The diode D2 protects the batteries against
discharge through U2 when the power source U1 is switched
off. For the 1N4001 diode, used in this design, the typical
forward voltage drop is 0.9 V. This value must be added to the
voltage of three fully charged battery cells in series when the
output voltage threshold level is chosen. Capacitor C3 is
used for frequency compensation of an internal
transconductance amplifier.
The circuit shown in Figure 2 provides high efficiency
battery charging with protection against short circuit
accomplished by the LM2575 internal current limiting.
Because it is possible to set the output voltage threshold by a
simple resistor divider, various types of battery cells, as well
as various number of cells, can be charged. Nonetheless,
this circuit has some limitations. In the high–side current
sensing configuration, shown in Figure 2, the VCC Pin 7 of the
MC33341 is connected to the output of the power supply
circuit. Such a configuration offers the advantage of a
common return path for both ICs, the LM2575–ADJ and the
MC33341, but it has also a drawback. The low limit of the
MC33341 supply voltage is 1.9 V. That implies that charging
MOTOROLA ANALOG IC APPLICATIONS INFORMATION
3
AN1593
Figure 3. Low Cost Switching Regulator Performs Constant–Current/Constant–Voltage 3 Cell Charging Function.
Version with Full Short–Circuit Regulation Control Capability.
ON/OFF
+
+Vin
1
Unregulated
DC – Input
Vin = 10 to 40 V
C1 +
100
m
F/50 V
U1
LM2575–ADJ
5
Output
2
4 Feedback
R4
1.0 k
W
C4
680 nF
Threshold Voltage
Control
(from Controller)
+VO
1N4001
R2
39
W
3
Gnd
D1
1N5819
L1
400
m
H
+
C2
330
m
F/16 V
ON/OFF
Control
(from Controller)
Gnd
D3
1N4745A
R5
560
W
R6
100
W
D2
A
1
8
7
2
S2
R3
27
W
C3
33 nF
6
+
–
3 Battery Cells
Under Charge
1
U2
MC33341
5
R1
10 k
W
–VO
2
3
4
1
2
S1
RS
0.22
W
Charge Current
Control
(from Controller)
Figure 4. Constant–Voltage/Constant–Current Charging Characteristic
of the Improved Current Source for Battery Chargers
6
VO, OUTPUT VOLTAGE (V)
5
4
3
2
1
0
0
0.4
0.2
0.6
0.8
1.0
IO, OUTPUT CURRENT (A)
Figure 4 shows the V/I charging characteristic of the
improved circuit (see schematic in Figure 3). The small
voltage drop in the beginning of the V/I characteristic is
caused by dynamic resistance of the diode D2. This
phenomenon can be eliminated either by reconnection of the
resistor R2 from the point “A” to the cathode of the diode D2,
or this diode might be replaced by shorting wire. Both actions
would cause also another effect on the V/I characteristic of
this circuit. When the output voltage drops below
approximately 1.2 V, the output current will fall down
accordingly, thus creating a typical “foldback” V/I
characteristic.
4
MOTOROLA ANALOG IC APPLICATIONS INFORMATION
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NOTES:
1. Inductor L1: Inductance Pins 4, 6. Manufacturer: TECH 39 Power Electronic,
Tel. +33/1–4115–1681, Fax +33/1–4409–5051.
2. All tolerances
±10%,
unless otherwise specified.
MOTOROLA ANALOG IC APPLICATIONS INFORMATION
Figure 5. PCB Layout Component Side
Performance of the Sample Design (Refer to the Circuit Shown in Figure 2)
Gnd
+Vin + C1
Input voltage range . . . . . . . . . . .
Battery charging current . . . . . . .
Open output voltage . . . . . . . . . .
Power converter efficiency . . . . .
C3
Component
L1
RS
U2
U1
D2
D1
C3
C2
C1
R4
R3
R2
R1
S1
L1
U1
R4
Table 1. Parts List (Refer to the Circuit Shown in Figure 2)
ON/OFF
SW1
U2
+
D1
C2
Current Control
Quantity
1
1
1
1
1
1
1
1
1
1
1
1
1
1
–VO
+VO
8.0 to 40 Vdc
0.88 A
5.45 V
77%, Vin = 24 V
400
µH,
1.6 A
0.22
Ω,
1/2 W
1.0 kΩ, 1/4 W
39 kΩ, 1/4 W
10 kΩ, 1/4 W
330
µF,
16 V
100
µF,
50 V
1.0 A, 100 V
Value/Rating
27
Ω,
1/4 W
1.0 A, 40 V
33 nF
–
–
–
Figure 6. PCB Layout Copper Side
Schottky Diode, 1N5819
Capacitor Electrolytic
Capacitor Electrolytic
Inductor, 77 458 BV
Capacitor Ceramic
IC, LM2575–ADJ
Diode, 1N4001
IC, MC33341
Description
Resistor
Resistor
Resistor
Resistor
Resistor
Switch
AN1593
5
R2
R3
D2
RS
R1
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