Datasheet

MCP73871
DS20002090C-page 28  2008-2013 Microchip Technology Inc.
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 guidances from
battery manufacturers. For example, a 1000 mAh
battery pack has a preferred fast charge current of
0.7C. Charging at 700 mA provides the shortest charge
cycle times without degradation to the battery pack
performance or life.
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:
EQUATION 6-1:
For example, power dissipation with a 5V, ±10% input
voltage source and 500 mA, ±10% fast charge current
is:
EXAMPLE 6-1:
This power dissipation with the battery charger in the
QFN-20 package causes thermal regulation to enter as
depicted. Alternatively, the 4 mm x 4 mm DFN package
could be utilized to reduce heat by adding vias on the
exposed pad.
6.1.1.3 External Capacitors
The MCP73871 device is stable with or without a
battery load. To maintain good AC stability in the Con-
stant Voltage mode, a minimum capacitance of 4.7 μF
is recommended to bypass the V
BAT
pin to V
SS
. This
capacitance provides compensation when there is no
battery load. In addition, the battery and
interconnections appear inductive at high frequencies.
These elements are in the control feedback loop during
Constant Voltage mode. Therefore, the bypass
capacitance may be necessary to compensate for the
inductive nature of the battery pack.
Virtually any good quality output filter capacitor can be
used, regardless of the capacitor’s minimum Effective
Series Resistance (ESR) value. The actual value of the
capacitor (and its associated ESR) depends on the
output load current. A 4.7 μF ceramic, tantalum or
aluminum electrolytic capacitor at the output is usually
sufficient to ensure stability for charge currents up to
1000 mA.
6.1.1.4 Reverse-Blocking Protection
The MCP73871 device provides protection from a
faulted or shorted input. Without the protection, a
faulted or shorted input would discharge the battery
pack through the body diode of the internal pass
transistor.
6.1.1.5 Temperature Monitoring
The charge temperature window can be set by placing
fixed value resistors in series-parallel with a thermistor.
The resistance values of R
T1
and R
T2
can be calculated
with the following equations to set the temperature win-
dow of interest.
For NTC thermistors:
EQUATION 6-2:
PowerDissipation V
DDMAX
V
PTHMIN
–I
REGMAX
=
Where:
V
DDMAX
= the maximum input voltage
I
REGMAX
= the maximum fast charge current
V
PTHMIN
= the minimum transition threshold
voltage
PowerDissipation 5.5V 2.7V–550mA 1.54W==
24k R
T1
R
T2
R
COLD

R
T2
R+
COLD
----------------------------------+=
5k R
T1
R
T2
R
HOT

R
T2
R+
HOT
-------------------------------+=
Where:
R
T1
= the fixed series resistance
R
T2
= the fixed parallel resistance
R
COLD
= the thermistor resistance at the
lower temperature of interest
R
HOT
= the thermistor resistance at the
upper temperature of interest