ST AN1284 Application note

V

A Li-Ion BATTERY CHARGER USING TSM102A

This application note explains how to use the
TSM102 in an S MPS-type battery charger wh ich
features :

❑ Voltage Control
❑ Current Control
❑ Low Battery Detection and E nd Of Charg

Detection

Figure 1 : TSM102 Pinout

AN1284

APPLICATION NOTE

by R. LIOU

1 - TSM102 PRESENTATION

The TSM102 integrated circuit includes two Oper­ational Amplifiers, two Comparators and one ad­justable precision Voltage Reference (2.5V to
36V, 0.4% or 1%).
TSM102 can sustain up to 36V power supply volt­age.

1

2

3

+

CC

5

6

7

Vref

2 - APPLICATION CONTEXT AND PRINCIPLE
OF OPERATION

In the battery charging field which requires ever in­creasing performances in more and more reduced
space, the TSM102A provides an attractive solu­tion in terms of PCB area saving, precision and
versatility.
Figure 2 shows the secondary side of a battery
charger (SMPS type) where TSM 102A is used in
optimised conditions : the two Operational Amplifi­ers perform current a nd voltage control, the two

Comparators provide “End of Charge” and “Low
Battery” signals and the Voltage Reference en­sures precise reference for all measurements.
The TSM102A is supplied by an auxiliary power
supply (forward configuration - D7) regulated by a
bipolar transistor and a zener diode on its base
(Q2 and DZ), and s moothed by the capacitors C3
and C4. R15 polarizes the base of the transistor
and at the same time limits the current through the

July 2000

COMP

TSM102

COMP

zener diode during regulation mode of the auxilia­ry power supply.

The current and voltage regulations are made
thanks to the two Operational Amplifiers.

The first amplifier senses the current flow through
the sense resistor Rs and compares it wit h a part
of the reference voltage (resistor bridge R7, R8,
R9). The second amplifier compares the reference
voltage with a part of t he charger’s output (resistor
bridge R1, R2, R3).

When either of these two operational amplifiers
tends to lower its ouput, this linear information is
propagated towards the primary side via two OR­ing diodes (D1, D2) and an optocoupler (D3). The
compensation loops o f these regulation functions
are ensured by the capacitors C1 and C2.

16

15

14

V

-

CC

12

11

10

Cathode

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AN1284 - APPLICATION NOTE

Figure 2 : The Application Schematic - Battery Charger Secondary Side

The first comparator ensures the “Low Battery”
signal generation thanks to the comparison of a
part of the charger’s output voltage (resistor
bridge R17, R19) and the reference voltage. Prop­er hysteresis is given thanks to R20. An improve­ment to the chargers security and to the b attery’s
life time optimization is achieved by lowering the
current control measurement thanks to Q1 that
shunts the resistor R9 when the battery’s voltage
is below the “Low Battery” level.
The second comparator ensures the “End of
Charge” signal generation thanks to the compari­son of a part of the charger’s output voltage (resis­tor bridge R1, R2, R3) and the reference voltage.
When either of these two signals is active, the cor­responding LED is polarized for convenient visual­ization of the battery status.

3 - CALCULATION OF THE ELEMENTS

All the components values have been chosen for a
two-Lithium-Ion batteries charge application :

❑ Current Control : 720mA (Low Battery cur-

rent control : 250mA)

❑ Voltage Control : 8.4V (= 2x 4.2V)
❑ Low Battery : 5.6V (= 2x 2.5V + 0.6V)

❑ End of Charge : 8.3V (= 2x 4.15V)

Current Control:

The voltage reference is polarized than ks to the
R4 resistor (2.5mA), and the cathode of the refer­ence gives a fixed 2.500V voltage.

I = U / R= [ Vref ( R8 + R9 ) / (R7 + R8 + R9) ] / Rs

= [ 2.5 x (390 + 820) / (10000 + 390 + 820) ] / 0.375
= 720mA

I = 720mA
P = power dissipation through the sense resistor = R I2 = 0.375 x

0.7202 = 194mW

In case of “Low Battery” conditions, the current
control is lowered thanks to the following equation:

I = U / R= = [ Vref R8 / (R7 + R8) ] / Rs

= [ 2.5 x 390 / (10000 + 390 ) ] / 0.375
= 250mA

I (LoBat t) = 250mA

Voltage Control:

Vout = Vref / [ R2 / (R1 + R2 + R3) ]

= 2.5 / [ 56 / (131.5 + 56 + 0. 68 ) ]
= 8.400V

Vout = 8.400V

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