ST AN1051 Application note

	AN1051
®	APPLICATION NOTE
	TSM102 : A DUAL LI-ION BATTERY CHARGER
	USING AN ST SILICON TRIPLET
	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. -type battery charger which features :

.Dual Li-Ion Battery charging with Constant Current/Constant Voltage

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

.Precision Voltage Control

.Temperature and Failing Battery protection End of Charge detection

1 - DEVICES PRESENTATION

The VIPER20 integrates on the same 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 chargers 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, Oscillator Safe Guard, 3 to 6 V power supply range and high ESD tolerance make the device well suited for noisy environment.

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

Figure 1 : ST62T25C, TSM102A and VIPER20 Pin
Vdd	1		28	Vss
TIMER	2		27	PA0
OSCin	3	ST62T25C	26	PA1	20mA
OSCout	4		25	PA2		1	TSM102	16
NMI	5		24	PA3		2		15			PowerSO-10		DRAIN
PC7	6		23	PA4		3		14
									OSC	6		5	SOURCE
							COMP	COMP
PC6	7		22	PA5
						VCC+		VCC-	Vdd	7	VIPER20	4	SOURCE
PC5	8		21	PA6		5		12	NC	8		3	NC
PC4	9		20	PA7		6		11	NC	9		2	SOURCE
TEST	10		19	PB0					COMP	10		1	SOURCE
						7		10
!RESET	11		18	PB1
	12		17	PB2		Vref		Cathode
PB7
PB6	13		16	PB3
PB5	14		15	PB4

DIP28 - SO28

Pentawatt-HV

VIPER20

	1	2			3	4			5
OSC									COMP
		Vdd			DRAIN		SOURCE

February 1999

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AN1051 - APPLICATION NOTE

2 - APPLICATION CONTEXT AND PRINCIPLE OF OPERATION

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 monitored.

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
I				is reached
V
Cell voltage rises							Occasional topping charge is
to voltabe limit							applied if battery voltage drops
15	30	45	60	75	90 105	120	135 150 165 180	minutes of charge
CONSTANT			1h CONSTANT			2h	NO TRICKLE
CURRENT CHARGE				VOLTAGE CHARGE			CHARGING
				70% charged			100% charged

Maximum charge current is applied until the set voltage limit is reached.

Charge current starts to drop as the battery gets saturated.

The Li-ion battery cannot absorb over-charge. Trickle charge would be harmfull.

During Stage 1, the battery is charged at constant 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 graphite electrode).

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

Thus during Stage 2 a constant voltage charge is applied. Battery chargers manufacturers recommend 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 solution in terms of performance, cost efficiency and versatility.

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AN1051 - APPLICATION NOTE

Figure 3a and 3b show the primary 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 MOSFET.

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

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