ST AN1051 Application note

AN1051

APPLICATION NOTE

TSM102 : A DUAL LI-ION BATTERY CHARGER

USING A N ST SILICON T RI PLE T

by A. BAILLY, D. CABROL, J. CAMIOLO

S. LAFFONT, R. LIOU

This application note explains how to use the VIPER20, the ST62 µController and the TSM102A in an SMPS-ty pe battery charger which features :

Dual Li-Ion Battery charging with Constant Current/Cons tant Voltage

Battery type recognition (4.1V or 4.2V and different capacities )

Precision V oltage Control

Temperature and Failing Battery protection

End of Charge detection

1 - DEVICES PRESENTATION

The VIPER20 integrates on the sa me chip a PWM circuit together with a high voltage avalanche rugged vertical MOSFET (600V, 0.5A) which make it ideal for primary side control of battery cha rgers or power supplies featuring up to 20W output. Moreover, this device allows stand-by mode operation without additional components.

The ST62T25C is a low cost 28 pins 8-bit Microcontroller available in Mask ROM, FastROM and OTP versions. It features an A/D converter with up to 16 channels, 20 I/O pins of which 4 have High Current capability . An integrated Static Reset circuitry , Oscillat or Safe Guard, 3 to 6 V power supply range and high ESD tolerance make the device well suited for noisy envir onment.

The TSM102A integrated circuit i ncludes two Operational Amplifiers (type LM358), two Comparators (typ e LM393) and one adjustable precision V oltage Reference (type TL1431 : 2.5V to 36V , 0.4% or 1%). TSM102A can sustain up to 36V power supply voltage.

Figure 1 : ST62T25C, TSM1 02A and VIPER20 Pin

Vdd

TIMER

OSCin

NMI

PC7

PC6

PC5

PC4

TEST

PB7

PB6

PB5

OSCout

!RESET

February 1999

ST62T25C

DIP28 - SO28

Vss

PA0

PA1

20mA

Vref

COMP

TSM102

COMP

PA2

PA3

PA4

PA5

PA6

PA7

PB0

PB1

PB2

PB3

PB4

Cathode

OSC

COMP

Pentawatt-HV

PowerSO-10

Vdd

VIPER20

DRAIN

SOURCE

VIPER20

234

OSC

COMP

Vdd

DRAIN

SOURCE

1/9

AN1051 - APPLICATION NOTE

2 - APPLICATION CONTEXT AND P RINCI PLE OF OP E RATION The Li-Ion Battery

Rechargeable battery using Lithium have high voltage, big capacity and light weight, yielding an extraordinary energy density, more than twice the one of the NiCd. The maximum load current is not as high as for the NiCd, but is still sufficient for many applications such as cellular phone and camcorder.

To improve lifetime and keep the battery in safe operating conditions, some protection circuitry is always added inside the battery pack that can disconnect the electrochemical cell from the external connectors. This protection circuitry is designed to trigger in case of overcurrent (both when charging and discharging), overvoltage (when charging) and undervoltage (when discharging). The cell temperature is also mo nitored.

Charging Principle

The charging principle of the Lithium-Ion batteries is very different from the Nickel type. Figure 2 shows the different stages in the charging process. Time values are only indicative and depend on battery type and speed of charge.

Figure 2 : Li-Ion Charging Scheme

Maximum cell voltage

Cell voltage rises to voltabe limit

is reached

Occasional topping charge is applied if battery voltage drops

15 30 45 60 75 90 105 120 135 150 165 180 minutes of charge

CONSTANT

CURRENT CHARGE

Maximum charge current

is applied until the set

voltage limit is reached.

CONSTANT VOLTAGE CHARGE

70% charged

Charge current starts

to drop as the battery

gets saturated.

NO TRICKLE CHARGING

100% charged

The Li-ion battery cannot

absorb over-charge. Trickle

charge would be harmfull.

During Stage 1, the battery is charged at consta nt current. The battery voltage is slowly increasing from original voltage up to the maximum cell voltage, which depends on the battery electrode technology (4.2V/cell for coke electrode, 4.1V /cell for gra phite electrode) .

Should this maximum voltage be exceeded and the battery could suffer significant damage and the protection circuitry may tri gger.

Thus during Stage 2 a constant voltage charge is applied. Battery chargers manufactur ers recom mend a highly precise voltage supervision of ±0.05 V/cell.

The current is slowly decreasing as the battery gets closer to full capacity . End of charge can be detected by the charge current getting lower than a fixed threshold value (usually

one fifth to one tenth of the constant current charge value).

The dual Li-Ion Batteries charger :

In this application which requires ever increasing performances in more and more reduced space, the silicon triplet VIPER20, ST62T25C and TSM102A provides an attractive soluti on in terms of performance, cost efficiency and versatility.

2/9

AN1051 - APPLICATION NOTE

Figure 3a and 3b show the pri mary and the secondary sides of the battery charger (SMPS type, primary and secondary sides) where the VIPER20, the ST62T25C and TSM102A are combined to ensure safe charging of two Li-Ion battery cells in parallel (main and spare batteries).

The Viper20 ensures with a very low component count the energy transfer from the off-line primary side to the secondary side thanks to its PWM ability ( with externally adjustable frequency of operation) and integrated high voltage avalanche-rugged vertical MOSFE T.

The ST62T25C µController is used to :

• recognize the Li-Ion battery type (4.1V or 4.2V and capacity)

• manage the charging of the two different cells in parallel thanks to the proper command of two power

switches

• prevent the battery charging in case of overtemperature or undertemperature

• drive adequate LEDs for convenient visual information

The TSM102A can ensure all analog interfacing between the batteries and the µP by

• controlling current and voltage with adequate feedback via the optocoupler to the primary side

• offering highly precise voltage reference for all measurements

• amplifying the current signal through the sense resistor to be monitored by the µController

• providing a low cost solution for 5V power supply of the MCU

Figure 3a : Primary Side of Battery Charger

3/9

+ 6 hidden pages