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Power Management
Texas Instruments Incorporated
Single-chip bq2403x power-path manager
charges battery while powering system
By Jinrong Qian (Email: jqian@ti.com)
Applications Manager, Battery Management Applications
The lithium-ion (Li-ion) battery is widely adopted in
portable devices because of its high energy density on
both a gravimetric and volumetric basis. Users of applica-
tions such as smartphones, PDAs, and MP3 players want
to be able to operate the device from an input source
without a battery. This requires a power architecture with
two separate paths for device system power and battery
charging, called “power-path management.”
Dynamic power-path-management (DPPM)
battery charger
In the most commonly used battery-charging and system-
power configuration, the system load is directly connected
to the battery-charger output. This architecture is simple
and low-cost but can cause improper charge termination
and false safety-timer warnings due to ineffective control
of the battery charge current.
The bq2403x family of DPPM battery chargers has a
power-sharing capability of simultaneously powering the
system and charging the battery. This eliminates the
charge-termination and safety-timer issues, minimizes the
ac adapter power rating, and improves system reliability. It
also allows the system to operate while charging a deeply
discharged battery.
Figure 1 is a block diagram of a simplified power-path-
management battery charger. When the ac adapter is
plugged in, MOSFET Q1 is used to preregulate the system
bus voltage, V OUT , which is higher than the maximum
battery regulation voltage, V BAT . This establishes a direct
path from the adapter input to the system. The MOSFET
Q2 is dedicated to charging the battery, so there is no
interaction between the battery and the system. When a
USB is present and selected, the MOSFET Q3 is fully
turned on, the Q3 output provides almost the same volt-
age as the USB output, and the MOSFET Q2 controls the
battery charging.
Figure 1. Simplified power-path-management
battery charger
V OUT
Adapter
Q1
USB
Q3
Q2
V BAT
Device
System
C 0
Battery
GND
DPPM actively monitors the system bus voltage. If the
system bus voltage drops to a preset value due to a limited
amount of input current from the adapter or USB, the
battery-charging current is reduced until the output voltage
stops dropping. The DPPM control tries to reach a steady-
state condition where the system gets its needed current
and the battery is charged with the remaining current.
Because of this, the adapter is designed based on average
power from the system, not on maximum peak system
power. This allows the designer to use a smaller power
rating and a cheaper adapter.
Figure 2 shows a typical DPPM applications circuit. When
the total current from the system and battery charger
exceeds the current limit of the ac adapter or USB, the
capacitors connected to the system bus start to discharge
and system bus voltage begins to drop. When the system
bus voltage drops to the predetermined threshold set by
the DPPM pin, the charge current is reduced to prevent a
system crash from overloading the ac adapter. If the system
12
Analog Applications Journal
High-Performance Analog Products
3Q 2006
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Texas Instruments Incorporated
Power Management
Figure 2. DPPM battery charger
bq2403x
V OUT
I OUT
AC Adapter
System Load
AC
OUT
USB
C0
10 µF
USB
OUT
R4
D1
D2
D3
STAT1
OUT
R5
D4
High
Enable
C2
10 µF
STAT2
CE
C1
10 µF
103AT
R6
V BAT
D5
BAT
USBPG
I CHG
R7
C4
10 µF
D6
RT1
0 Ω
ACPG
BAT
TS
High: 500 mA
Low: 100 mA
ISET2
3.3 V/20 mA
R1
1.33 k Ω
R2
51.1 k
ISET1
LDO
Fast Charge Current: 800 mA
Temperature: 0 to 45°C
Fast Charge Time Limit: 5 hours
V = 4.0 V
R4, R5, R6, and R7: 2 k
C3
1µF
High: AC
Low: USB
RT2
Open
TMR
PSEL
Ω
VSS
DPPM
DPPM
Ω
R3
34.8 k Ω
bus voltage cannot be maintained even when the charge
current is reduced to 0 A, the battery will temporarily
discharge and provide power to the system to avoid a
system crash. This is called “battery supplement mode”
and is shown in Figure 3 along with the DPPM experi-
mental waveforms.
The DPPM voltage threshold, V DPPM , is set by resistor
R3 and is typically below the regulation voltage at the
OUT pin to safely keep the system operating. R3 is
calculated by
Figure 3. DPPM experimental waveforms
V
= 4.26 V
V AC (1 V/div)
V OUT ( 1 V/div)
DPPM
DPPM
Mode
DPPM
Mode
Battery Supplement
Mode
I
I CHG (1 A/div)
115
100
.
×
V
DPPM
R
3
=
.
0V
0A
µ
A
I
I OUT (1 A/div)
R1 is used to set the fast charge current and is given by
450
×
2 5
.
V
R
1
=
.
I CHG
R2 is used to set the safety-timer value. Typically the tem-
perature qualification for allowing a Li-ion battery to be
charged is between 0ºC and 45ºC. RT1 and RT2 are pro-
grammed for different temperature ranges.
The battery charger can select either ac or USB power
as the main power source through the PSEL pin, and max-
imum current is also selectable through ISET2 when the
USB port is selected.
Three power MOSFETs and a power controller are inte-
grated in a thermally enhanced 3.5 × 4.5-mm QFN package.
A thermal regulation loop reduces the charge current to
prevent the silicon temperature from getting higher than
125ºC. Whenever the charge current is reduced either by
active thermal regulation or by active DPPM, the safety-
timer duration is automatically increased to prevent an
unexpected false safety-timer warning. Charge termina-
tion is also disabled when either DPPM or the thermal
regulation loop is active. This approach prevents false
charge termination.
Conclusion
DPPM reduces the battery-charging current while powering
the system load when the system bus voltage drops to a
predetermined threshold due to limited input current. DPPM
also completely eliminates the issues of battery and system
interaction such as false charge termination and false safety-
timer warnings. The DPPM battery charger is ideal for
power systems that simultaneously charge the battery.
Related Web sites
13
Analog Applications Journal
3Q 2006
High-Performance Analog Products
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