FAIRCHILD AN-9721

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 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<2.7V). Charging is therefore still
possible by driving current into the pack through Q2’s body
diode. When FAN540X determines that VBAT<2.0V, it uses
a 30mA linear current source to charge the battery beyond
2.0V before applying the full ICHARGE in PWM mode.
The FAN5402 and FAN5405 continue charging after VBUS
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 IIN
limit drops to 100mA for about 1ms, unless IINLIM is 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
JEITA1 standards 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 VOREG tolerance over
temperature of +1% (42mV), a setting of 4.20V would
produce a worst-case VFLOAT of 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.
2.
3.
4.
5.
6.
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 VBAT down 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 VBAT is rising and attempts to return to CV mode.
Some overshoot can occur (about 50mV, worst-case) while
the CV voltage loop regains control.
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 ICHARGE
current used when IINLIM is set to No Limit.
Without software mitigation, this overshoot should be
subtracted from 4.25V to determine the maximum VFLOAT
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 VBAT up 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, RSENSE is 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 VFLOAT of 3.54V for t15MIN
(nominally 12 minutes, 15 minutes maximum). If there is no
battery when VBUS first 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 t32S timer (minimum of 18 seconds) begins counting.
If t32S expires without being reset, all registers reset and
charging continues with default settings in t15MIN mode. The
processor should write a 1 to the TMR_RST bit at least
every 15 seconds.
1
A Guide to the Safe Use of Secondary Lithium Ion Batteries in Notebooktype Personal Computers, Japan Electronics and Information Technology
Industries Association and Battery Association of Japan, April 20, 2007.
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 12/23/10
Set the OTG pin HIGH. When VBUS is plugged in,
IINLIM is set to 500mA until the system processor powers
up and can set parameters through I2C.
Program the Safety Register
Set IINLIM to 11 (No Limit).
Set OREG to the desired value (typically 4.18).
Reset the IOLEVEL bit, then set IOCHARGE.
Set IINLIM to 500mA if a USB source is connected or any
other level that is preferred.
www.fairchildsemi.com
2
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
register can only be programmed after either:
Program VFLOAT by 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.
ƒ Power is first applied to the IC by plugging in a battery
with sufficient charge to run the processor
or
ƒ VBUS is plugged in, a battery is in place, and no I2C
writes occurred before writing to the SAFETY register.
Setting the Charge Current (IOCHARGE)
Most battery manufacturers recommend the battery be
charged at a rate not to exceed 1C. For example, an 800mAHr 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 RSENSE). 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 RSENSE remains 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 IINLIMIT bits based on
the connected power source:
Table 1.
Preventing Charging at
Temperature Extremes
Input Current Limit
Power Source
USB 1.0
USB 2.0
(2)
USB 3.0
Wall Wart
IINLIM
00
01
10
11
The JEITA specification prohibits charging below a
minimum temperature (typically 0°C) and above a maximum
temperature (typically 60°C). Full current and rated VFLOAT
charging is only allowed inside an even more narrow range
(typically above +10°C and below 45°C). The allowable
temperature, VFLOAT, and charge currents should be specified
by the battery manufacturer.
100mA
500mA
800mA
No Limit
Note:
2. The USB 3.0 maximum available configured current is
900mA.
The FAN540X IC’s can automatically charge when VBUS
comes up. The default charge current is limited to 340mA
and default VFLOAT is limited to 3.54V, which is within the
boundaries of the reduced ICHARGE and VFLOAT for 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,
VBUS begins to drop. The special charger loop scales back
the charging current to prevent VBUS from 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.
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 12/23/10
The temperature sensing IC can be powered from either
PMID (which is protected from high-voltage excursions) or
from VREG, if the IC can run from a 1.8V supply.
www.fairchildsemi.com
3
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
6K
100K
U1A
10K
VREG
U1B
100K
S-5842ADAAQ-I6T1G
DISABLE
FAN5403
DISABLE
4.7K
β = 3500
Figure 3.
Temperature Limit IC Inhibits Charging
Below 0°C and Above 60°C
2K
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
DISCLAIMER
FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS
HEREIN TO IMPROVE RELIABILITY, FUNCTION, OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE
APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS
PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
LIFE SUPPORT POLICY
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.
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 12/23/10
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.
www.fairchildsemi.com
4