Datasheet

MCP73827
DS21704B-page 10 © 2007 Microchip Technology Inc.
5.0 DETAILED DESCRIPTION
Refer to the typical application circuit, Figure 6-1.
5.1 Analog Circuitry
5.1.1 CHARGE CURRENT MONITOR (I
MON
)
The I
MON
pin provides an output voltage that is propor-
tional to the battery charging current. It is an amplified
version of the sense resistor voltage drop that the cur-
rent loop uses to control the external P-channel pass
transistor. This voltage signal can be applied to the
input of an A/D Converter and used by a host microcon-
troller to display information about the state of the bat-
tery or charge current profile.
5.1.2 CELL VOLTAGE MONITORED INPUT
(V
BAT
)
The MCP73827 monitors the cell voltage at the V
BAT
pin. This input is tied directly to the positive terminal of
the battery. The MCP73827 is offered in two fixed-volt-
age versions for single cells with either coke or graphite
anodes: 4.1V (MCP73827-4.1) and 4.2V
(MCP73827-4.2).
5.1.3 GATE DRIVE OUTPUT (V
DRV
)
The MCP73827 controls the gate drive to an external
P-channel MOSFET, Q1. The P-channel MOSFET is
controlled in the linear region, regulating current and
voltage supplied to the cell. The drive output is auto-
matically turned off when the input supply falls below
the voltage sensed on the V
BAT
input.
5.1.4 CURRENT SENSE INPUT (V
SNS
)
Fast charge current regulation is maintained by the
voltage drop developed across an external sense resis-
tor, R
SENSE
, applied to the V
SNS
input pin. The follow-
ing formula calculates the value for R
SENSE
:
Where:
V
CS
is the current limit threshold
I
OUT
is the desired peak fast charge current in
amps. The preconditioning current is scaled to
approximately 43% of I
PEAK
.
5.1.5 SUPPLY VOLTAGE (V
IN
)
The V
IN
input is the input supply to the MCP73827. The
MCP73827 automatically enters a power-down mode if
the voltage on the V
IN
input falls below the voltage on
the V
BAT
pin. This feature prevents draining the battery
pack when the V
IN
supply is not present.
5.2 Digital Circuitry
5.2.1 SHUTDOWN INPUT (SHDN)
The shutdown input pin, SHDN
, can be used to termi-
nate a charge anytime during the charge cycle, initiate
a charge cycle, or initiate a recharge cycle.
Applying a logic High input signal to the SHDN
pin, or
tying it to the input source, enables the device. Apply-
ing a logic Low input signal disables the device and ter-
minates a charge cycle. In shutdown mode, the
device’s supply current is reduced to 0.5 µA, typically.
5.2.2 CHARGE STATUS OUTPUT (MODE)
A charge status output, MODE, provides information on
the state of charge. The open drain output can be used
to illuminate an external LED. Optionally, a pull-up
resistor can be used on the output for communication
with a microcontroller. Table 5-1 summarizes the state
of the charge status output during a charge cycle.
TABLE 5-1: Charge Status Output.
R
SENSE
V
CS
I
OUT
------------=
Charge Cycle State Mode
Qualification OFF
Preconditioning ON
Controlled Current Fast Charge ON
Constant Voltage OFF
Disabled - Sleep mode OFF
Battery Disconnected OFF