Datasheet
MCP73837/8
DS22071B-page 20 © 2007-2011 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 6-2
and Figure 6-3 depict the accompanying charge
profile.
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 in the shortest charge cycle time without
degradation of a battery's life and performance.
6.1.1.2 Thermal Considerations
The worst-case power dissipation in the battery char-
ger 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
THERMV
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
100
110
120
130
140
150
Time (Minutes)
Battery Voltage (V)
0
0.2
0.4
0.6
0.8
1
1.2
Charge Current (A)
V
DD
= 5.2V
R
PROG
= 1 kΩ
1200 mAh Li-Ion Battery
I
OUT
V
OUT
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
012345678910
Time (Minutes)
Battery Voltage (V)
0
0.3
0.6
0.9
1.2
Charge Current (A)
V
DD
= 5.2V
R
PROG
= 1 k
Ω
1200 mAh Li-Ion Battery
V
OUT
I
OUT