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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

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.

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

and comparing it to a fixed value set by the bridge

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

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 :

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.

VCC

VREF

CSEN

CRREF

GND

CRIN

OUT

VRIN

1.24V

1.4mA

1

2

3

45

6

7

8

V

OFF

R

⋅

=

AN1283

APPLICATION NOTE

A BATTERY CHARGER USING TSM101

May 2001

by S. LAFFONT and R. LIOU