A BATTERY CHARGER USING THE TSM101
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 : TSM101 Schematic Diagram
1 Vref 8
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 feedback
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
A great majority of low or medium end power
supplies is voltage regulated by using shunt pro-
grammable voltage references like the TL431
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
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
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
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.
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
R2 and R3 can be chosen using the following
R2 = R3 x
(Vref − Vr5)
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 :
• Vr4 = Io x R4 with Io = 1.4mA
This can be used to lower the charging current or
eventually to stop the charge, if Vr4 > Vr5
In our example, the current offset is equal to 700 -
200mA = 500mA, representing a voltage offset
Vr4 = 150mV across R4.
The following values are used on the application
• 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
Figure 3 : SMPS Using the TSM101