AN-9721 Datasheet, PDF (1/4 Pages) Fairchild Semiconductor – Li-Ion Battery Charging Basics

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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 higher-
capacity 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 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.
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.
The protection circuit’s resistance should be considered to
be part of the battery’s total ESR.
CELL
ESR
+
PROTECTION
CIRCUIT
Q1
Q2
+
CONTROL
–
Figure 1. Li-Ion Battery Pack
During charging, assuming the battery was not too deeply
discharged, a constant current I_CHARGE is 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
register setting.
When V_BAT, the voltage at the battery terminals, reaches
V_FLOAT, I_CHARGE is limited by the cell voltage, V_CELL:
I_CHARGE
=
V_BAT − V_CELL
R_ESR
(1)
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.
V_ORVEGFLOAT
1^ICOCCHuArRreGnEt
I_CHARGE
V ^BAT
V_SHV_OSRHTORT
II_{SPRHEOCRHTARGE}
I_TERM
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 12/23/10
PRE- CURRENT REGULATION
VOLTAGE
CHARGE
REGULATION
Figure 2. Li-Ion Charge Profile
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