© 2007 Microchip Technology Inc. DS21704B-page 13
MCP73827
V
INMIN
is the minimum input voltage source
I
OUT
is the maximum peak fast charge current
R
SENSE
is the sense resistor
Worst case, V
GS
with a 5V, +/-10% input voltage
source, 100 mΩ, 1% sense resistor, and a maximum
sink voltage of 1.6V is:
At this worst case V
GS
, the R
DSON
of the MOSFET
must be low enough as to not impede the performance
of the charging system. The maximum allowable
R
DSON
at the worst case V
GS
is:
The Fairchild NDS8434 and International Rectifier
IRF7404 both satisfy these requirements.
6.1.1.3 EXTERNAL CAPACITORS
The MCP73827 is stable with or without a battery load.
In order to maintain good AC stability in the constant
voltage mode, a minimum capacitance of 10 µF is rec-
ommended to bypass the V
BAT
pin to GND. This capac-
itance 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 nec-
essary to compensate for the inductive nature of the
battery pack.
Virtually any good quality output filter capacitor can be
used, independent of the capacitor’s minimum ESR
(Effective Series Resistance) value. The actual value of
the capacitor and its associated ESR depends on the
forward trans conductance, g
m
, and capacitance of the
external pass transistor. A 10 µF tantalum or aluminum
electrolytic capacitor at the output is usually sufficient
to ensure stability for up to a 1 A output current.
6.1.1.4 REVERSE BLOCKING PROTECTION
The optional reverse blocking protection diode
depicted in Figure 6-1 provides protection from a
faulted or shorted input or from a reversed polarity input
source. Without the protection diode, a faulted or
shorted input would discharge the battery pack through
the body diode of the external pass transistor.
If a reverse protection diode is incorporated in the
design, it should be chosen to handle the peak fast
charge current continuously at the maximum ambient
temperature. In addition, the reverse leakage current of
the diode should be kept as small as possible.
6.1.1.5 SHUTDOWN INTERFACE
In the stand-alone configuration, the shutdown pin is
generally tied to the input voltage. The MCP73827 will
automatically enter a low power mode when the input
voltage is less than the output voltage reducing the bat-
tery drain current to 8 µA, typically.
By connecting the shutdown pin as depicted in
Figure 6-1, the battery drain current may be further
reduced. In this application, the battery drain current
becomes a function of the reverse leakage current of
the reverse protection diode.
6.1.1.6 CHARGE STATUS INTERFACE
The charge status indicator, MODE, can be utilized to
illuminate an LED when the MCP73827 is in the con-
trolled current phase. When the MCP73827 transitions
to constant voltage mode, the MODE pin will transition
to a high impedance state. A current limit resistor
should be used in series with the LED to establish a
nominal LED bias current of 10 mA. The maximum
allowable sink current of the MODE pin is 30 mA.
6.2 PCB Layout Issues
For optimum voltage regulation, place the battery pack
as close as possible to the device’s V
BAT
and GND
pins. It is recommended to minimize voltage drops
along the high current carrying PCB traces.
If the PCB layout is used as a heatsink, adding many
vias around the external pass transistor can help con-
duct more heat to the back-plane of the PCB, thus
reducing the maximum junction temperature.
V
GS
1.6V 4.5V 758mA 99mΩ×–()– 2.8– V==
R
DSON
V
INMIN
I
PEAK
R
SENSE
×– V
BATMAX
–
I
OUT
----------------------------------------------------------------------------------------------=
DSON
4.5V 758mA 99mΩ×– 4.242V–
758mA
-------------------------------------------------------------------------------- 242mΩ==
