AN1283 Datasheet, PDF (1/7 Pages) STMicroelectronics – A BATTERY CHARGER USING TSM101

AN1283
APPLICATION NOTE
A BATTERY CHARGER USING TSM101
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
1.24V
VREF
1
VCC
8
CSEN
2
1.4mA
CRREF
3
VRIN
7
OUT
6
GND
4
CRIN
5
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
resistor.
A great majority of low or medium end power
supplies is voltage regulated by using shunt
programmable voltage references like the TL431
(Figure 2).
The galvanic insulation of the control information
is done by using an optocoupler in linear mode
with a variable photo current depending on the
difference between the actual output voltage and
the desired one.
by S. LAFFONT and R. LIOU
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
limitation loop use an internal 1.24V band-gap
voltage reference. This voltage reference has a
good precision (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
increases 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.
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 ⋅ I₀
with I₀ = 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.
May 2001
1/7