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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-cadmi-

um 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 sup-

plies is voltage regulated by using shunt program-

mable 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 dif-

ference 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 regula-

tion of the output current of the SMPS and a pre-

cise 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.

1

2

3

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8

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7

Vref

APPLICATION NOTE

A BATTERY CHARGER USING THE TSM101