AN-9721 [FAIRCHILD]

Li-Ion Battery Charging Basics; 锂离子电池的充电基础知识


型号：	AN-9721
厂家：	FAIRCHILD SEMICONDUCTOR
描述：	Li-Ion Battery Charging Basics 锂离子电池的充电基础知识电池
文件：	总4页 (文件大小：124K)
中文：	中文翻译
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www.fairchildsemi.com

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 higher-

capacity batteries. These higher-capacity batteries require

high-current charging solutions, which can best be served

with efficient PWM chargers.

PROTECTION

CIRCUIT

CELL

ESR

–

CONTROL

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.

Figure 1.

Li-Ion Battery Pack

During charging, assuming the battery was not too deeply

discharged, a constant current I_CHARGEis provided until the

battery’s voltage has risen to V_FLOAT. The maximum float

voltage is typically specified by the battery manufacturer

and is programmed into the charger IC through the OREG

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 over-

charging 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.

When V_BAT, the voltage at the battery terminals, reaches

_FLOAT, I_CHARGEis limited by the cell voltage, V_CELL:

V_BAT−V_CELL

(1)

I_CHARGE

R_ESR

As the internal cell voltage rises to approach V_BAT, the

charge current continues to decrease until it reaches a

termination current, which is commonly set for 10% of the

full charge current.

OREG

FLOAT

I_CHARGE

Note:

1C Current

OCHARGE

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.

I_TERM

SHORT

The protection circuit’s resistance should be considered to

be part of the battery’s total ESR.

SHORT

PRECHARGE

PRE-

CHARGE

CURRENT REGULATION

VOLTAGE

REGULATION

Figure 2.

Li-Ion Charge Profile

Rev. 1.0.0 • 12/23/10

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 V_BATto 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 (V_CELL<2.7V). Charging is therefore still

possible by driving current into the pack through Q2’s body

diode. When FAN540X determines that V_BAT<2.0V, it uses

a 30mA linear current source to charge the battery beyond

2.0V before applying the full I_CHARGEin PWM mode.

The FAN5402 and FAN5405 continue charging after V_BUS

POR with the default parameters, regulating the VBAT line

to 3.54V until the host processor issues commands or the 15

minute timer expires. In this way, the FAN5402/05 can start

the system without a battery.

The FAN5400 family’s soft-start function can interfere with

the system supply with battery absent. The soft-start

activates whenever VOREG, IINLIM, or IOCHARGE are

set from a lower to higher value. During soft-start, the I_IN

limit drops to 100mA for about 1ms, unless I_INLIMis set to

11 (no limit). This could cause the system processor to fail

to start. To avoid this behavior, use the following sequence:

Avoid Over-Voltage

JEITA¹standards require that the battery voltage not exceed

4.25V. While battery manufacturers may suggest that the

cell should be charged to 4.20V, the charging IC’s tolerance

should be taken into account. With a V_OREGtolerance over

temperature of +1% (42mV), a setting of 4.20V would

produce a worst-case V_FLOATof 4.242V. This allows no

room for temporary excursions above the OREG setting,

which can occur during large system load transient events,

such as a GSM pulse release.

1. Set the OTG pin HIGH. When VBUS is plugged in,

_INLIMis set to 500mA until the system processor powers

up and can set parameters through I²C.

2. Program the Safety Register

When charging a battery that’s already in CV (constant

voltage) charge with a high current, a 2A GMS pulse loads

the battery and drives V_BATdown by about 500mV. This

causes the charger IC to change from CV to CC (constant

current) control, providing about 1.2A of current at the

highest setting (FAN540x). When the GSM pulse stops, the

full 1.2A current flows into the battery briefly while the IC

senses that V_BATis rising and attempts to return to CV mode.

Some overshoot can occur (about 50mV, worst-case) while

the CV voltage loop regains control.

3. Set I_INLIMto 11 (No Limit).

4. Set OREG to the desired value (typically 4.18).

5. Reset the IOLEVEL bit, then set IOCHARGE.

6. Set I_INLIMto 500mA if a USB source is connected or any

other level that is preferred.

During the initial system startup, while the charger IC is being

programmed, the system current is limited to 340mA for 1ms

during steps 4 and 5. This is the value of the soft-start I_CHARGE

current used when I_INLIMis set to No Limit.

Without software mitigation, this overshoot should be

subtracted from 4.25V to determine the maximum V_FLOAT

voltage. The overshoot can, however, be mitigated in

software, which is discussed later in this document.

If the system powers up without a battery present, the CV bit

should be set. When a battery is inserted, the CV bit clears.

Programming Charge Parameters

System Startup

The following recommendations are for general guidance

only. For the correct charge parameter values, refer to the

manufacturer’s recommended charging conditions for the

specific battery in use.

Typically, systems run from the battery. If the battery is

missing or deeply discharged, the charger needs to be able to

automatically and safely bring V_BATup to a point where the

system processor can wake up and manage battery charging.

The FAN5403 and FAN5405 feature automatic charging.

For the settings below, R_SENSEis assumed to be 68mΩ.

Watchdog Timer

When a charger is connected and a battery is present, the

FAN5403 begins charging the battery without processor

intervention with its default V_FLOATof 3.54V for t1_5MIN

(nominally 12 minutes, 15 minutes maximum). If there is no

battery when V_BUSfirst becomes valid, the FAN5403 detects

Once the processor has powered up, charging continues

under processor control. As soon as the processor writes to

I²C, the t_32Stimer (minimum of 18 seconds) begins counting.

If t_32Sexpires without being reset, all registers reset and

charging continues with default settings in t1_5MINmode. The

processor should write a 1 to the TMR_RST bit at least

every 15 seconds.

¹A Guide to the Safe Use of Secondary Lithium Ion Batteries in Notebook-

type Personal Computers, Japan Electronics and Information Technology

Industries Association and Battery Association of Japan, April 20, 2007.

Rev. 1.0.0 • 12/23/10

www.fairchildsemi.com

AN-9721

APPLICATION NOTE

Safety First

Programming the Float Voltage (OREG)

The first register that should be programmed after the

processor wakes up is the SAFETY register. The SAFETY

Program V_FLOATby setting OREG, following the battery

manufacturer’s recommended maximum float voltage, but

subtracting 40mV for the charger IC’s OREG tolerance.

Ensure that the overshoot does not exceed the 4.25V level

specified in the JEITA standard. Typically, programming

OREV to 4.16V should suffice.

Power is first applied to the IC by plugging in a battery

with sufficient charge to run the processor

V_BUSis plugged in, a battery is in place, and no I²C

writes occurred before writing to the SAFETY register.

Setting the Charge Current (I_OCHARGE

)

Most battery manufacturers recommend the battery be

charged at a rate not to exceed 1C. For example, an 800mA-

Hr battery can be charged with up to 800mA of current,

which allows it to charge in about one hour.

If the battery is removed during charging with the TE bit set,

the SAFETY register is continually reset every two seconds.

Once a battery is inserted, the SAFETY register should be

the first register programmed.

The FAN540X limits the charging current for unattended

charging to 340mA (23.1mV across R_SENSE). To achieve the

desired charge current, set IOCHARGE (Reg4[6:4]) for the

desired charging current, then reset the IO_LEVEL bit

(Reg5[4]).

Input Power Source

The amount of power that can be drawn from a USB source

is determined after a negotiation with the USB equipment.

Until that negotiation takes place, 100mA is the maximum

current allowed. The OTG pin allows the USB transceiver to

set the maximum current during unattended charging. When

OTG is HIGH, the input power source is limited to 500mA

during unattended charging. When OTG is LOW, the

FAN540X limits its input current to 100mA.

Termination Settings

The termination current is typically set for ~10% of the

charge current. If the system load is connected at VBAT,

nominal system load current should be added to the battery

termination current. If the TE bit is set, when the voltage

across R_SENSEremains below the ITERM setting for 32ms,

charging stops. For example, with an 800mA-Hr battery and

a 200mA maximum system load, ITERM should be set for

300mA.

After the processor takes control, it can determine whether

the power source is USB or a dedicated charger (“wall

wart”). Typically the charger can supply more current than

the 500mA allowed by USB. Set the I_INLIMITbits based on

the connected power source:

Preventing Charging at

Temperature Extremes

Table 1. Input Current Limit

Power Source

I_INLIM

The JEITA specification prohibits charging below a

minimum temperature (typically 0°C) and above a maximum

temperature (typically 60°C). Full current and rated V_FLOAT

charging is only allowed inside an even more narrow range

(typically above +10°C and below 45°C). The allowable

temperature, V_FLOAT, and charge currents should be specified

by the battery manufacturer.

USB 1.0

USB 2.0

100mA

500mA

800mA

No Limit

USB 3.0⁽²⁾

Wall Wart

Note:

2. The USB 3.0 maximum available configured current is

900mA.

The FAN540X IC’s can automatically charge when V_BUS

comes up. The default charge current is limited to 340mA

and default V_FLOATis limited to 3.54V, which is within the

boundaries of the reduced I_CHARGEand V_FLOATfor batteries

that are inside the wider temperature range of 0°C to 60°C.

Some wall warts have limited power. The processor is

typically unable to determine this until after charging starts.

If the wall wart is unable to support the charging current,

V_BUSbegins to drop. The special charger loop scales back

the charging current to prevent V_BUSfrom dropping lower

than 4.53V, which draws as much current as the wall-wart is

capable of producing, if required.

If the battery temperature is outside the 0°C to 60°C,

charging can be inhibited by using the DISABLE pin with a

low-cost temperature switch IC.

The processor can determine if special charger loop is active

by reading the SP bit.

The temperature sensing IC can be powered from either

PMID (which is protected from high-voltage excursions) or

from V_REG, if the IC can run from a 1.8V supply.

Rev. 1.0.0 • 12/23/10

www.fairchildsemi.com

AN-9721

APPLICATION NOTE

Seiko Instrument’s S-5842ADAAQ-I6T1G temperature

sensor provides a logic 1 on its open-drain DETL pin which,

when connected to the FAN54xx’s DISABLE pin with a

pull-up resistor to VREG, inhibits charging when outside the

allowable temperature range.

If a thermistor is provided inside the battery pack, the circuit

in Figure 4 raises the DISABLE when battery temperature is

outside the 0°C to 60°C.

VREG

11K

PMID

100K

U1A

U1B

VREG

10K

S-5842ADAAQ-I6T1G

FAN5403

DISABLE

4.7K

β = 3500

Figure 3.

Temperature Limit IC Inhibits Charging

Below 0°C and Above 60°C

Figure 4.

Disabling Charge at Temperature

Extremes Using a Thermistor

Some battery vendors allow some charging outside the

JEITA-recommended temperature range if charge current,

time, and voltage are restricted. Consult the battery vendor

for safe charging recommendations.

Related Datasheets

FAN5400 Family

FAN5420 Family.

S-5842A series datasheet, Seiko Instruments: http://www.sii-ic.com

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PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.

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FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS

WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION.

As used herein:

1. Life support devices or systems are devices or systems

which, (a) are intended for surgical implant into the body, or

(b) support or sustain life, or (c) whose failure to perform

when properly used in accordance with instructions for use

provided in the labeling, can be reasonably expected to

result in significant injury to the user.

2. A critical component is any component of a life support

device or system whose failure to perform can be

reasonably expected to cause the failure of the life support

device or system, or to affect its safety or effectiveness.

Rev. 1.0.0 • 12/23/10

www.fairchildsemi.com

AN-9721 [FAIRCHILD]

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