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MCP73837/8
DS22071A-page 20
2007 Microchip Technology Inc.
6.0
APPLICATIONS
The MCP73837/8 devices are designed to operate in
conjunction with a host microcontroller or in stand-
alone applications. The MCP73837/8 devices provide
the preferred charge algorithm for Lithium-Ion and
Lithium-Polymer cells Constant-current followed by
Constant-voltage.
Figure 6-1 depicts a typical stand-
alone MCP73837 application circuit, while
Figure 6-2profile.
FIGURE 6-1:
MCP73837 Typical Stand-Alone Application Circuit.
FIGURE 6-2:
Typical Charge Profile
(1200 mAh Li-Ion Battery).
FIGURE 6-3:
Typical Charge Profile in
Thermal Regulation (1200 mAh Li-Ion Battery).
6.1
Application Circuit Design
Due to the low efficiency of linear charging, the most
important factors are thermal design and cost, which
are a direct function of the input voltage, output current,
and thermal impedance between the battery charger
and the ambient cooling air. The worst-case situation is
when
the
device
has
transitioned
from
the
Preconditioning mode to the Constant Current mode. In
this situation, the battery charger has to dissipate the
maximum power. A trade-off must be made between
the charge current, cost, and thermal requirements of
the charger.
6.1.1
COMPONENT SELECTION
Selection of the external components in
Figure 6-1 is
crucial to the integrity and reliability of the charging
system. The following discussion is intended as a guide
for the component selection process.
6.1.1.1
Charge Current
The preferred fast charge current for Lithium-Ion cells
should always follow references and guidance from
battery manufacturers. For example, programming
700 mA fast charge current for a 1000 mAh Li-Ion
battery pack if its preferred fast charge rate is 0.7C.
This will result the shortest charge cycle time without
degradation a battery's life and performance.
6.1.1.2
Thermal Considerations
The worst-case power dissipation in the battery
charger occurs when the input voltage is at the
maximum and the device has transitioned from the
Preconditioning mode to the Constant-current mode. In
this case, the power dissipation is:
STAT1
V
AC
V
SS
/PG
V
BAT
Single
Li-Ion
Cell
4
MCP73837
5
3
1
2
STAT2
THERM
V
USB
PROG1
PROG2
USB Port
6
7
Hi
Low
Thermsitor
R
PROG
8
9
10
1
ΚΩ
1
ΚΩ
1
ΚΩ
REGULATED
WALL CUBE
C
IN1
C
IN2
C
OUT
0.0
1.0
2.0
3.0
4.0
5.0
0
10
20
30
40
50
60
70
80
90
10
0
11
0
12
0
13
0
14
0
15
0
Time (Minutes)
Ba
tt
ery
Volt
age
(V
)
0
0.2
0.4
0.6
0.8
1
1.2
Charge
Current
(A)
VDD = 5.2V
RPROG = 1 k
1200 mAh Li-Ion Battery
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0123456789
10
Time (Minutes)
Ba
tt
e
ry
Vo
lt
ag
e(V
)
0
0.3
0.6
0.9
1.2
Charge
C
u
rrent
(
A
)
VDD = 5.2V
RPROG = 1 k
1200 mAh Li-Ion Battery