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Power Management
Texas Instruments Incorporated
Battery-charger front-end IC improves
charging-system safety
By Mao Ye,
Applications Engineer, Battery Management Applications
Introduction
Battery-powered portable devices such as cellular phones
have become an important part of people’s daily lives in
the past few years. Many types of adapters are available to
charge the lithium-ion (Li-ion) battery and power the sys-
tem, and their electrical specifications usually differ from
one manufacturer to another. This challenges system
designers to build portable devices that will meet safety
and reliability requirements when used with different
adapters. This article describes a new battery-charger
front-end (CFE) IC, the Texas Instruments (TI) bq243xx,
which is optimized to improve the safety of charging Li-ion-
powered systems. Together with the battery-charger IC
and the protection module in a battery pack, a charging
system using the bq243xx CFE provides more robust
system-level protection.
Main safety concerns in charging systems
Damage to the charging system can occur due to input
overvoltage, input overcurrent, battery overvoltage, or
reverse input voltage.
Input overvoltage can be caused by hot-plugging an
adapter or using the wrong adapter; or by a transient or
steady-state overvoltage condition. The most common
occurrences are from hot plugging a charged, unregulated,
or incorrect adapter; or from load transients. The unregu-
lated adapter under no load will charge the adapter’s
output capacitor to the peak rectified AC voltage, about
1.4 times the rated DC voltage. This is often an issue with
“low-voltage-process” (7-V-process) ICs. Figure 1 shows
the output voltage of a typical regulated adapter versus an
unregulated adapter.
Input-overcurrent challenges are not an issue with
stand-alone chargers, since their constant-current mode
limits the amount of current delivered to the output or
battery. However, with power-path-management parts,
which have a direct connection from the input to the
system bus voltage, there is often no protection from
excessive current draw. Lately there has been some
concern over the safety of operating adapters in their
current-limit mode and a desire for a programmable input-
current-limit circuit to ensure that the adapter does not
get into this mode.
Li-ion and Li-polymer battery packs are known for the
potentially dangerous “flaming” condition that can occur if
they are overcharged under high temperature. The key
indication of overcharging is excessive cell voltage. To
improve battery safety, many manufacturers are adding
second-level overvoltage protection to remove the input
power source when battery overvoltage is detected.
With universal connectors, it is a concern that an
adapter with reverse polarity will be connected to the
input. Without input reverse-polarity protection, the para-
sitic diode between the substrate and the IC will become
forward-biased, causing a malfunction or damage to the
IC. The two basic solutions to achieving input reverse-
polarity protection are shown in Figure 2. The first solution
is to add a diode in series with the input to block any
reverse current. However, this will increase the power
dissipation. The second solution is to use a low-r
DS(on)
MOSFET in series with the input to minimize the power
dissipation.
Figure 2. Input-voltage reverse-
polarity protection
Figure 1. Regulated- and
unregulated-adapter load lines
+
Battery
Charger
INPUT
8
5-V/500-mA Unregulated
–
6
(a) Series diode solution
4
5-V/500-mA Regulated
+
–
2
Battery
Charger
INPUT
0
0
0.5
1
1.5
2
Adapter Output Current (A)
(b) MOSFET solution
14
Analog Applications Journal
High-Performance Analog Products
www.ti.com/aaj
2Q 2008
Texas Instruments Incorporated
Power Management
Figure 3. Typical application circuit with CFE
bq243xx
IN
OUT
Battery
Charger
bq24080
1
8
Q1
Q2
+
CFE
Controller
Adapter
or USB
V
BAT
3
System
6
PGATE
–
2
5
V
SS
Disable/
Enable
CE
CFE solution for improving battery-charger safety
Figure 3 shows a typical circuit for a battery-charging
system with a bq243xx CFE. The CFE protects the system
from input overvoltage by isolating high input voltage from
the low-voltage charger and system. The bq243xx family
offers a soft-start function to avoid inrush current and can
provide input-current regulation and protection, output-
voltage limiting/regulation, or battery overvoltage protec-
tion. Also available are optional features such as PGATE to
drive an external FET for reverse-polarity protection;
fault-status indication; a programmable input-current
limit; and enable/disable input power.
Figure 4 shows the typical response of the bq24314 CFE
to input overvoltage. The internal MOSFET switch imme-
diately turns off with less than a 1-µs delay once the input
voltage reaches the predetermined input-overvoltage
threshold.
When the system load exceeds the input-current limit,
the CFE activates the input-current regulation loop and
provides the maximum current limit set by the CFE. At a
certain overcurrent blanking time, the CFE will turn off
the MOSFET and may enter hiccup mode or latch mode
after overcurrent protection is activated 15 times, depend-
ing on the IC version. The typical hiccup and latch
response of the bq24314 CFE to input overcurrent is
shown in Figure 5.
Figure 4. Protective response of bq24314 to
input overvoltage
V
(5 V/div)
IN
1
V
OUT
(2 V/div)
2
FAULT (2 V/div)
4
1 ms/div
Figure 5. Protective response of bq24314 to
input overcurrent
Current (1 A/div)
3
V
OUT
(2 V/div)
2
FAULT (5 V/div)
4
200 ms/div
15
Analog Applications Journal
2Q 2008
www.ti.com/aaj
High-Performance Analog Products
Power Management
Texas Instruments Incorporated
Another important function of the CFE is to achieve
battery second-level overvoltage protection for improved
safety, although the battery pack itself achieves cell over-
voltage protection by turning off the protection MOSFET
in series with the cell. When the battery is overcharged
due to any failure of the battery charger or protection
MOSFET, the CFE will turn off its output with a 176-µs
delay time and will recover when the battery is no longer
experiencing overvoltage. The typical response of the
bq24314 CFE to battery overvoltage is shown in Figure 6.
Summary
The CFE can significantly improve the safety of battery-
operated systems by fully integrating protection from
input overvoltage, input overcurrent, battery overvoltage,
and reverse input polarity.
Related Web sites
power.ti.com
www.ti.com/sc/device/
partnumber
Replace partnumber with
bq24080,
bq24300,
bq24304,
or
bq24314
Figure 4. Protective response of bq24314 to
battery overvoltage
V
(2 V/div)
BAT
1
V
OUT
(2 V/div)
2
4
FAULT (2 V/div)
100 µs/div
16
Analog Applications Journal
High-Performance Analog Products
www.ti.com/aaj
2Q 2008
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