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AN-9721 Li-Ion Battery Charging Basics, Featuring the FAN5400 / FAN5420 Family of PWM Battery Chargers Overview Today’s cell phones and other handheld devices provide ever increasing functionality and a richer user experience. As their functionality increases, the demand for battery power increases as well, which leads to adoption of highercapacity batteries. These higher-capacity batteries require high-current charging solutions, which can best be served with efficient PWM chargers. Lithium-Ion battery charging is simplified with modern IC charging solutions. This application note provides a guide for how to use the FAN5400 and FAN5420 family of PWM chargers for high-current, fast-charging solutions to minimize the charging time while providing full compliance to modern battery safety specifications.

Lithium-Ion Battery Charging Basics A Li-Ion battery charger must provide a constant current to the battery until the battery voltage has reached its “float” voltage. The battery can be thought of as a very large capacitor in series with a small resistance that represents its ESR (equivalent series resistance). Inside every battery pack is a protection IC, which features two back-to-back MOSFETs and an analog control circuit that prevents overcharging and over-discharging by monitoring the cell voltage and discharge current. The protection circuit is referred to as “secondary protection” because the charging system must also ensure that the battery is not overcharged. The protection circuit provides a back-up safety circuit where overcharging is concerned.

CELL

Q1

ESR

+

CONTROL

–

Figure 1.

When VBAT, the voltage at the battery terminals, reaches VFLOAT, ICHARGE is limited by the cell voltage, VCELL: ICHARGE =

VBAT − VCELL

(1)

RESR

As the internal cell voltage rises to approach VBAT, the charge current continues to decrease until it reaches a termination current, which is commonly set for 10% of the full charge current. VOREG V

ICHARGE

IOCHARGE 1C Current

T

V BA

ITERM

IIPRECHARGE SHORT

PRECHARGE

CURRENT REGULATION

Figure 2. © 2010 Fairchild Semiconductor Corporation Rev. 1.0.0 • 12/23/10

Li-Ion Battery Pack

During charging, assuming the battery was not too deeply discharged, a constant current ICHARGE is provided until the battery’s voltage has risen to VFLOAT. The maximum float voltage is typically specified by the battery manufacturer and is programmed into the charger IC through the OREG register setting.

VSHORT VSHORT

The protection circuit’s resistance should be considered to be part of the battery’s total ESR.

Q2

+

FLOAT

Note: 1. For functional clarity, Q1 and Q2 are shown as PMOS MOSFETs in series with the positive leg in Figure 1. Most protection circuits use NMOS MOSFETs in the return leg instead for lower cost.

PROTECTION CIRCUIT

VOLTAGE REGULATION

Li-Ion Charge Profile www.fairchildsemi.com

AN-9721

APPLICATION NOTE

Once the termination current is set (assuming charge termination has been enabled by setting the TE bit), the charger IC stops charging and waits for VBAT to discharge to a recharge threshold. For the FAN540x family, this threshold is 120mV below the OREG setting.

the battery’s absence and shuts down, preventing the system from running without a battery. This is useful when the system does not have another method of determining battery absence, since the charger typically cannot support GSM pulses or other high-load current events without a battery.

Deeply Discharged Cells

Running without a Battery

Q2 in the protection circuit is open if the cell was deeply discharged (VCELL