MCP73861/2/3/4
Figure 6-1 depicts a typical application circuit with
connection of the THERM input. The resistor values of
RT1 and RT2 are calculated with the following
equations.
5.0
DETAILED DESCRIPTION
5.1
Analog Circuitry
For NTC thermistors:
5.1.1
BATTERY MANAGEMENT INPUT
SUPPLY (VDD1, VDD2
)
2 × RCOLD × RHOT
----------------------------------------------
RT1
=
=
The VDD input is the input supply to the MCP7386X.
The MCP7386X automatically enters a Power-down
mode if the voltage on the VDD input falls below the
UVLO voltage (VSTOP). This feature prevents draining
the battery pack when the VDD supply is not present.
R
COLD – RHOT
2 × RCOLD × RHOT
----------------------------------------------
COLD – 3 × RHOT
RT2
R
For PTC thermistors:
5.1.2
PROG INPUT
2 × RCOLD × RHOT
----------------------------------------------
Fast charge current regulation can be scaled by placing
a programming resistor (RPROG) from the PROG input
to VSS. Connecting the PROG input to VSS allows for a
maximum fast charge current of 1.2A, typically. The
minimum fast charge current is 100 mA, set by letting
the PROG input float. The following formula calculates
RT1
=
=
R
HOT – RCOLD
2 × RCOLD × RHOT
----------------------------------------------
HOT – 3 × RCOLD
RT2
R
Where:
the value for RPROG
:
RCOLD and RHOT are the thermistor
resistance values at the temperature window
of interest.
13.2 – 11 × IREG
----------------------------------------
12 × IREG – 1.2
RPROG
=
Applying a voltage equal to VTHREF/3 to the THERM
input disables temperature monitoring.
where:
IREG = the desired fast charge current in amps.
5.1.5
TIMER SET INPUT (TIMER)
RPROG = measured in kΩ.
The TIMER input programs the period of the safety
timers by placing a timing capacitor (CTIMER) between
the TIMER input pin and VSS. Three safety timers are
programmed via the timing capacitor.
The preconditioning trickle-charge current and the
charge termination current are scaled to approximately
10% and 8% of IREG, respectively.
The preconditioning safety timer period:
5.1.3
CELL TEMPERATURE SENSOR
BIAS (THREF)
CTIMER
------------------
× 1.0Hours
tPRECON
=
0.1μF
A 2.5V voltage reference is provided to bias an external
thermistor for continuous cell temperature monitoring
and prequalification. A ratio metric window comparison
is performed at threshold levels of VTHREF/2 and
The fast charge safety timer period:
CTIMER
------------------
× 1.5Hours
tFAST
=
VTHREF/4.
0.1μF
5.1.4
CELL TEMPERATURE SENSOR
INPUT (THERM)
The elapsed time termination period:
CTIMER
------------------
× 3.0Hours
tTERM
=
The MCP73861/2/3/4 continuously monitors tempera-
ture by comparing the voltage between the THERM
input and VSS with the upper and lower temperature
0.1μF
The preconditioning timer starts after qualification and
resets when the charge cycle transitions to the fast
charge, Constant-current mode. The fast charge timer
and the elapsed timer start once the MCP7386X
transitions from preconditioning. The fast charge timer
resets when the charge cycle transitions to the
Constant-voltage mode. The elapsed timer will expire
and terminate the charge if the sensed current does not
diminish below the termination threshold.
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 (VDD).
The temperature-sensing circuit is removed from the
system when VDD is not applied, eliminating additional
discharge of the battery pack.
During thermal regulation, the timer is slowed down
proportional to the charge current.
© 2005 Microchip Technology Inc.
DS21893C-page 15
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