MCP73827
DS21704B-page 12 © 2007 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-chan-
nel pass transistor, Q1, and the ambient cooling air.
The worst-case situation is when the output is shorted.
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 is
crucial to the integrity and reliability of the charging sys-
tem. The following discussion is intended as 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 with-
out degradation to the battery pack performance or life.
The current sense resistor, R
SENSE
, is calculated by:
Where:
V
CS
is the current limit threshold voltage
I
OUT
is the desired fast charge current
For the 500 mAH battery pack example, a standard
value 100 mΩ, 1% resistor provides a typical peak fast
charge current of 530 mA and a maximum peak fast
charge current of 758 mA. Worst case power dissipa-
tion in the sense resistor is:
A Panasonic ERJ-L1WKF100U 100 mΩ, 1%, 1 W
resistor is more than sufficient for this application.
A larger value sense resistor will decrease the peak
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 peak fast charge current. The selected P-
channel MOSFET must satisfy the thermal and electri-
cal design requirements.
Thermal Considerations
The worst case power dissipation in the external pass
transistor occurs when the input voltage is at the maxi-
mum and the output is shorted. In this case, the power
dissipation is:
Where:
V
INMAX
is the maximum input voltage
I
OUT
is the maximum peak fast charge current
K is the foldback current scale factor.
Power dissipation with a 5V, +/-10% input voltage
source, 100 mΩ, 1% sense resistor, and a scale factor
of 0.43 is:
Utilizing a Fairchild NDS8434 or an International Recti-
fier IRF7404 mounted on a 1in
2
pad of 2 oz. copper, the
junction temperature rise is 90°C, approximately. This
would allow for a maximum operating ambient temper-
ature of 60°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 main-
taining 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
charge current is at the maximum. The worst case, V
GS
is:
Where:
V
DRVMAX
is the maximum sink voltage at the V
DRV
output
R
SENSE
V
CS
I
OUT
------------=
PowerD issipation 100mΩ 758mA
2
× 57.5mW==
PowerDissipation V
INMAX
I
OUT
K××=
PowerDissipation 5.5V 758mA× 0.43× 1.8W==
V
GS
V
DRVMAX
V
INMIN
I
OUT
R
SENSE
×)–(–=
