MCP73841/2/3/4
DS21823D-page 16 2003-2013 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 external P-channel
pass transistor and the ambient cooling air. The worst-
case situation occurs when the device has transitioned
from the preconditioning phase to the constant-current
phase. In this situation, the P-channel pass transistor
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 are
crucial to the integrity and reliability of the charging
system. The following discussion is intended to be a
guide for the component selection process.
6.1.1.1 Sense Resistor
The preferred fast charge current for Lithium-Ion cells
is at the 1C rate, with an absolute maximum current at
the 2C rate. For example, a 500 mAh battery pack has
a preferred fast charge current of 500 mA. Charging at
this rate provides the shortest charge cycle times
without degradation to the battery pack performance or
life.
The current sense resistor (R
SENSE
) is calculated by:
For the 500 mAh battery pack example, a standard
value 220 m, 1% resistor provides a typical fast
charge current of 500 mA and a maximum fast charge
current of 551 mA. Worst-case power dissipation in the
sense resistor is:
A Panasonic
®
ERJ-6RQFR22V, 220 mW, 1%, 1/8W
resistor in a standard 0805 package is more than
sufficient for this application.
A larger value sense resistor will decrease the fast
charge current and power dissipation in both the sense
resistor and external pass transistor, but will increase
charge cycle times. Design trade-offs must be
considered to minimize space while maintaining the
desired performance.
6.1.1.2 External Pass Transistor
The external P-channel MOSFET is determined by the
gate-to-source threshold voltage, input voltage, output
voltage and fast charge current. Therefore, the
selected P-channel MOSFET must satisfy the thermal
and electrical design requirements.
Thermal Considerations
The worst-case power dissipation in the external pass
transistor occurs when the input voltage is at the
maximum and the device has transitioned from the
preconditioning phase to the constant-current phase. In
this case, the power dissipation is:
Power dissipation with a 5V, ±10% input voltage
source, 220 m, 1% sense resistor is:
Utilizing a Fairchild™ NDS8434 or an International
Rectifier IRF7404 mounted on a 1in
2
pad of 2 oz.
copper, the junction temperature rise is 75°C,
approximately. This would allow for a maximum
operating ambient temperature of 75°C.
By increasing the size of the copper pad, a higher ambi-
ent temperature can be realized, or a lower value
sense resistor could be utilized.
Alternatively, different package options can be utilized
for more or less power dissipation. Again, design trade-
offs should be considered to minimize size while
maintaining the desired performance.
Electrical Considerations
The gate-to-source threshold voltage and R
DSON
of the
external P-channel MOSFET must be considered in the
design phase.
The worst-case V
GS
provided by the controller occurs
when the input voltage is at the minimum and the fast
charge current regulation threshold is at the maximum.
The worst-case V
GS
is:
R
SENSE
V
FCS
I
REG
------------
=
Where:
I
REG
is the desired fast charge current.
PowerDissipation 220m 551mA
2
66.8mW==
PowerDissipation V
DDMAX
V
PTHMIN
–I
REGMAX
=
Where:
V
DDMAX
is the maximum input voltage.
I
REGMAX
is the maximum fast charge current.
V
PTHMIN
is the minimum transition threshold voltage.
PowerDissipation 5.5V 2.75V–551mA 1.52W==
V
GS
V
DRVMAX
V
DDMIN
V
FCSMAX
––=
Where:
V
DRVMAX
is the maximum sink voltage at the
V
DRV
output
V
DDMIN
is the minimum input voltage source
V
FCSMAX
is the maximum fast charge current
regulation threshold
