®
AN896
APPLICATION NOTE
TSM101 USED IN A BATTERY CHARGER
by S. LAFFONT
This technical note shows how to use the TSM101
integrated circuit with a switching mode power
supply (SMPS) to realize a battery charger.
An example of realization of a 12V Nickel-cadmium
battery charger is given.
1 - TSM101 PRESENTATION
The TSM101 integrated circuit incorporates a high
stability series band gap voltage reference, two
ORed operational amplifiers and a current source
(Figure 1)
Figure 1 :
TSM101 Schematic Diagram
A great majority of low or medium end power
supplies is voltage regulated by using shunt pro-
grammable voltage references like the TL431
(Figure 2).
The galvanic insulation of the control information is
done by using an opto-coupler in linear mode with
a variable photo current depending on the differ-
ence between the actual output voltage and the
desired one.
A current limitation is used to protect the power
supply against short circuits, but lacks precision.
This limitation is generally realized by sensing the
current of the power transistor, in the primary side
of the SMPS.
The role of the TSM101 is to make a fine regulation
of the output current of the SMPS and a precise
voltage limitation.
The primary current limitation is conserved and
acts as a security for a fail-safe operation if a
short-circuit occurs at the output of the charger.
2 - PRINCIPLE OF OPERATION
The current regulation loop and the voltage limita-
tion loop use an internal 1.24V band-gap voltage
reference. This voltage reference has a good pre-
cision (better than 1.5%) and exhibits a very stable
temperature behavior.
The current limitation is performed by sensing the
voltage across the low ohmic value resistor R5 and
comparing it to a fixed value set by the bridge
composed by R2 and R3 (Figure 3).
When the voltage on R5 is higher than the voltage
on R3 the output of the current loop operational
amplifier decreases. The optocoupler current in-
creases and tends to reduce the output voltage by
the way of the PWM controller.
The voltage regulation is done by comparing a part
of the output voltage (resistor bridge R6, R7 and
P1) to the voltage reference (1.24V).
If this part is higher than 1.24V, the output of the
voltage loop operational amplifier decreases.
This IC compares the DC voltage and the current
level at the output of a switching power supply to
an internal reference.It provides a feedb ack
through an optocoupler to the PWM controller IC in
the primary side.
The controlled current generator can be used to
modify the level of current limitation by offsetting
the information coming from the current sensing
resistor.
February 1999
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AN896 - APPLICATION NOTE
Figure 2 :
SMPS Using a TL431 as Voltage Controller
The optocoupler current increases and tends to
reduce the output voltage by the way of the PWM
controller.
By enabling the TSM101 current source (pin 2) it is
possible to offset the current sensing by a voltage
equal to :
•
V
off
# R4 * Io with I
o
= 1.4mA
This offset lowers the output charge current and
this function can be used to charge two types of
batteries having different capacities. The current
source is enabled by connecting pin 2 to ground
3 - CALCULATION OF THE ELEMENTS
The charge current is regulated at 700mA (if the
charge control input is left open) or 200mA (if the
charge control input is put to ground ), allowing the
charge of two different types of batteries.
3.1 - Voltage limitation
The end-of- charge voltage is limited at 1.45V/cell,
this is the recommended voltage for an ambient
temperature at 25
o
C.
A diode is generally inserted at the output of the
charger to avoid the discharge of the battery if the
charger is not powered. This diode is sometimes
directly integrated in the battery pack. The influ-
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ence of this diode on the charge is negligible if the
voltage drop (0.7V) is taken into account during the
design of the charger.
The voltage at the output of the charger is :
•
V
out
=
R6+R7
R6
xV
r
and regarding R6 and R7 :
•
R6
= (
V
ref
)
x R7
V
out
−
V
ref
P1, which is a part of R6 and R7 is not considered
in this equation.
The following values are used on the application
board :
•
R7 = 12kΩ
•
R6 = 1kΩ
•
P1 = 220Ω, adjust for V
output
= 15.2V with the
battery replaced by a 1kΩ resistor
•
R10 = short circuit
•
C3 = 100nF
3.2 - Current regulation
R5 is the sense resistor used for current measure-
ment.
AN896 - APPLICATION NOTE
The current regulation is effective when the voltage
drop across R5 is equal to the voltage on pin 5 of
the TSM101 (assuming that the internal current
source is disabled).
For medium currents (<1A), a voltage drop across
R5 of 200mV = Vr5 is a good value, R5 can be
realized with standard low cost 0.5W resistors in
parallel.
•
R5
=
V
r5
, R5 = 0.285Ω (four 1.2Ω resistor in
I
ch
parallel)
R2 and R3 can be chosen using the following
formula :
•
R2
=
R3
x
In our example, the current offset is equal to 700 -
200mA = 500mA, representing a voltage offset
V
r4
= 150mV across R4.
The following values are used on the application
board :
•
R5 = 4 *1.2Ω 0.5W in parallel
•
R4 = 130Ω
•
R2 = 1.2kΩ
•
R3 = 220Ω
•
R9 = short circuit
•
R1 = 10kΩ
•
C2 = 100nF
•
C5 = 100nF
•
C1 = output capacitor of the SMPS
•
C4 = 10µF
HIGH FREQUECY COMPENSATION
Two R-C devices (R9+C2 & R10+C3) are used to
stabilize the regulation at high frequencies. The
calculation of these values is not easy and is a
function of the transfer function of the SMPS.
A guess value for the capacitors C2 and C3 is
100nF.
(
V
ref
−
V
r5
)
V
r5
CHARGE CONTROL
If the pin 2 is left open, the charge current is nominal
at # 700mA.
If pin 2 is connected to ground, the internal current
source is enabled, the current measurement is
off-setted by a voltage equal to :
•
V
r4
= I
o
x R4 with Io = 1.4mA
This can be used to lower the charging current or
eventually to stop the charge, if V
r4
> V
r5
Figure 3 :
SMPS Using the TSM101
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AN896 - APPLICATION NOTE
4 - SCHEMATIC DIAGRAM
Figure 2 represents a schematic of the output
circuit of a ”classical” SMPS using a TL431 for
voltage regulation. This circuit is modified to use
the TSM101 and the final circuit is represented in
figure 3.
5 - IMPROVEMENT
In applications requiring low voltage battery charge
or when the charger is in current regulation mode,
the output voltage can be too low to supply correctly
the TSM101.
The same problem occurs when the output is short-
circuited.
A solution to provide a quasi constant supply volt-
age to the TSM101 is shown at figure 4 : an auxiliary
Figure 4
winding is added at the secondary side of the
transformer.
This winding is forward coupled to the primary
winding, the voltage across it is directly proportional
to the mains rectified voltage, even if the flyback
voltage is close to zero.
As this auxiliary winding is a voltage source, it is
necessary to add a resistor (R11) on the cathode
of the rectifier (D3) to limit the current.
A low cost regulator (Q1 and Zener diode D4) is
used to power the TSM101. This is necessary with
autoranging SMPS with wide input voltages, for
example 90 to 240V without switching.
In standard SMPS with voltage ranges from 200 to
240VAC or 100 to 130VAC, this regulator can be
removed.
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the
consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from
its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications
mentioned in this publication are subject to change without notice. This publ ication supersedes and replaces all information
previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems
without express written approval of STMicroelectronics.
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