ST AN2407 Application note

AN2407

Application note

800mA standalone linear Li-Ion battery charger with thermal regulation

Introduction

One way to minimize the size and complexity of a battery charger is to use a linear-type charger. The linear charger drops the AC adapter voltage down to the battery voltage. The number of external components is low: linear chargers require input and output bypass capacitors, and sometimes need an external pass transistor, and resistors for setting voltage and current limits.

The main pitfall of a linear charger is power dissipation. The charger simply drops the AC adapter voltage down to the battery voltage.

In the case of an 800mA charger, a 5V±10% regulated AC adapter voltage, and battery voltage that varies between 4.2V and 2.5V, the power dissipation can range from 0.6W to 2.0W.

This type of charger is simpler than the switch-mode type, mainly because the passive LC filter is not required. It dissipates the most power when the battery voltage is at its minimum, since the difference between the fixed input voltage and the battery voltage is greatest during this condition.

Application diagram

September 2006

Rev 2

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Contents	AN2407

Contents

1	STBC08 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 3
2	Stability considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	4
3	Board layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
4	External components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
5	Power dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
6	Automatic recharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
7	CHRG and Power-on status output pins . . . . . . . . . . . . . . . . . . . . . . . . .	9
8	USB and wall adapter power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
9	Charge current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
	9.1 Programming charge current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
	9.2 Maximum charge current in temperature . . . . . . . . . . . . . . . . . . . . . . . . .	11

Appendix A Board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

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AN2407	STBC08 description

1 STBC08 description

The STBC08 is a constant current/constant voltage charger for single cell Li-Ion battery. No external sense resistor or blocking diode is required and its MLPD 3x3mm² 6L package make it ideally suited for portable applications.

The STBC08 is designed to comply with USB power specifications. An internal block regulates the current when the junction temperature increases in order to protect the device when it operates in high power or high ambient temperature.

The maximum power dissipation occurs when VBAT is 2.9V with the maximum charge current.

The charge voltage is fixed at 4.2V, and the charge current limitation can be programmed using a single resistor connected between pins PROG and GND. The charge cycle finishes when the current flowing to the battery is 1/10 of the programmed value. If the external adaptor is removed, the STBC08 switches off and only 2µA can flow from the battery to the device. The device can be put into Shutdown Mode, reducing the supply current to 25µA.

Figure 1. Block diagram

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Stability considerations	AN2407

2 Stability considerations

The STBC08 contains two control loops: constant voltage and constant current. The constant-voltage loop is stable without any compensation when a battery is connected with low impedance leads. Excessive lead length, however, may add enough series inductance to require a bypass capacitor of at least 1µF from BAT to GND. Furthermore, a 4.7µF capacitor with a 0.2Ω to 1Ω series resistor from BAT to GND is required to keep ripple voltage low when the battery is removed.

High value capacitors with very low ESR (especially ceramic) reduce the constant-voltage loop phase margin.

Ceramic capacitors up to 22µF may be used in parallel with a battery, but larger ceramics should be decoupled with 0.2Ω to 1Ω of series resistance.

In constant-current mode, the PROG pin is in the feedback loop, not the battery. Because of the additional pole created by PROG pin capacitance, capacitance on this pin must be kept to a minimum. With no additional capacitance on the PROG pin, the charger is stable with program resistor values as high as 12k. However, additional capacitance on this node reduces the maximum allowed program resistor.Therefore, if the PROG pin is loaded with a capacitance, CPROG, the following equation should be used to calculate the maximum resistance value for RPROG:

Equation 1

	1
----------------------------------------------------------
RPROG ≤
2π • 5 • 105 • CPROG

Average, rather than instantaneous, battery current may be of interest to the user. For example, if a switching power supply operating in low current mode is connected in parallel with the battery, the average current being pulled out of the BAT pin is typically of more interest than the instantaneous current pulses. In such a case, a simple RC filter can be used on the PROG pin to measure the average battery current as shown in Figure 2.

This design includes a 20kΩ resistor between the PROG pin and the filter capacitor to ensure stability (CFILTER = 100nF).

Figure 2. Isolating capacitive load on PROG pin and filtering

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AN2407	Board layout considerations

3 Board layout considerations

Due to the small size of the MLP package, it is very important to use a good thermal PC board layout to maximize the available charge current. The thermal path for the heat generated by the IC is from the die to the copper lead frame, through the package leads, (especially the ground lead) to the PC board copper. The PC board copper is the heat sink. The footprint copper pads should be as wide as possible and expand out to larger copper areas to spread and dissipate the heat to the surrounding ambient. Feed-through vias to inner or backside copper layers are also useful in improving the overall thermal performance of the charger. Other heat sources on the board, not related to the charger, must also be considered when designing a PC board layout because they will affect overall temperature rise and the maximum charge current.

Table 1 lists thermal resistance for several different board sizes and copper areas.

Table 1.	Measured thermal resistance (2-layer board)
		Copper area		Board area	Thermal resistance
					junction-to-ambient
	Top		Bottom
2500mm2			2500mm2	2500mm2	125°C/W
1000mm2			2500mm2	2500mm2	125°C/W
225mm2			2500mm2	2500mm2	130°C/W
100mm2			2500mm2	2500mm2	135°C/W
50mm2			2500mm2	2500mm2	150°C/W

Appendix A: Board layout on page 12 contains an illustration of the complete assembly board.

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