2003-2013 Microchip Technology Inc. DS21823D-page 13
MCP73841/2/3/4
5.0 DETAILED DESCRIPTION
5.1 Analog Circuitry
5.1.1 CHARGE CURRENT SENSE INPUT
(SENSE)
Fast charge current regulation is maintained by the
voltage drop developed across an external sense
resistor (R
SENSE
) applied to the SENSE input pin. The
following formula calculates the value for R
SENSE
:
The preconditioning trickle-charge current and the
charge termination current are scaled to approximately
10% and 7% of I
REG
, res pec t ive l y.
5.1.2 BATTERY MANAGEMENT INPUT
SUPPLY (V
DD
)
The V
DD
input is the input supply to the MCP7384X.
The MCP7384X automatically enters a power-down
mode if the voltage on the V
DD
input falls below the
undervoltage lockout voltage (V
STOP
). This feature
prevents draining the battery pack when the V
DD
supply is not present.
5.1.3 CELL TEMPERATURE SENSOR
BIAS (THREF)
A 2.55V voltage reference is provided to bias an
external thermistor for continuous cell temperature
monitoring and pre-qualification. A ratio metric window
comparison is performed at threshold levels of
V
THREF
/2 and V
THREF
/4. Cell temperature monitoring
is provided by both the MCP73841 and MCP73842.
5.1.4 CELL TEMPERATURE SENSOR
INPUT (THERM)
The MCP73841 and MCP73842 continuously monitor
temperature by comparing the voltage between the
THERM input and V
SS
with the upper and lower
temperature thresholds. A negative or positive
temperature coefficient (NTC or PTC) thermistor and
an external voltage divider typically develop this
voltage. The temperature-sensing circuit has its own
reference, to which it performs a ratio metric
comparison. Therefore, it is immune to fluctuations in
the supply input (V
DD
). The temperature-sensing circuit
is removed from the system when V
DD
is not applied,
eliminating additional discharge of the battery pack.
Figure 6-1 depicts a typical application circuit with
connection of the THERM input. The resistor values of
R
T1
and R
T2
are calculated with the following
equations.
For NTC thermistors:
For PTC thermistors:
Applying a voltage equal to 0.85V to the THERM input
disables temperature monitoring.
5.1.5 TIMER SET INPUT (TIMER)
The TIMER input programs the period of the safety
timers by placing a timing capacitor (C
TIMER
) between
the TIMER input pin and V
SS
. Three safety timers are
programmed via the timing capacitor.
The preconditioning safety timer period:
The fast charge safety timer period:
The elapsed time termination period:
The preconditioning timer starts after qualification and
resets when the charge cycle transitions to the con-
stant-current, fast charge phase. The fast charge and
elapsed timers start once the MCP7384X transitions
from preconditioning. The fast charge timer resets
when the charge cycle transitions to the constant-volt-
age phase. The elapsed timer will expire and terminate
the charge if the sensed current does not diminish
below the termination threshold.
R
SENSE
V
FCS
I
REG
------------
=
where:
I
REG
is the desired fast charge current in amps
R
T1
2 R
COLD
R
HOT
R
COLD
R
HOT
–
----------------------------------------------
=
R
T2
2 R
COLD
R
HOT
R
COLD
3 R
HOT
–
----------------------------------------------
=
R
T1
2 R
COLD
R
HOT
R
HOT
R
COLD
–
----------------------------------------------
=
R
T2
2 R
COLD
R
HOT
R
HOT
3 R
COLD
–
----------------------------------------------
=
where:
values at the temperature window of interest.
R
COLD
and R
HOT
are the thermistor resistance
t
PRECON
C
TIMER
0.1F
-------------------1.0Hour s=
t
FAST
C
TIMER
0.1F
-------------------1.5Hours=
t
TERM
C
TIMER
0.1F
-------------------3.0Hours=
