MCP73853/55
5.0
5.1
5.1.1
DETAILED DESCRIPTION
Analog Circuitry
BATTERY MANAGEMENT INPUT
SUPPLY (V
DD1
, V
DD2
)
connection of the THERM input. The resistor values of
R
T1
and R
T2
are calculated with the following
equations.
For NTC thermistors:
2
×
R
COLD
×
R
HOT
R
T1
= ----------------------------------------------
R
COLD
–
R
HOT
2
×
R
COLD
×
R
HOT
R
T2
= ----------------------------------------------
R
COLD
–
3
×
R
HOT
For PTC thermistors:
2
×
R
COLD
×
R
HOT
R
T1
= ----------------------------------------------
R
HOT
–
R
COLD
2
×
R
COLD
×
R
HOT
R
T2
= ----------------------------------------------
R
HOT
–
3
×
R
COLD
Where:
R
COLD
and R
HOT
are the thermistor
resistance values at the temperature window
of interest.
Applying a voltage equal to V
THREF
/3 to the THERM
input disables temperature monitoring.
The V
DD
input is the input supply to the MCP7385X
devices. The MCP7385X devices automatically enter a
power-down mode if the voltage on the V
DD
input falls
below the UVLO voltage (V
STOP
). This feature prevents
draining the battery pack when the V
DD
supply is not
present.
5.1.2
PROG INPUT
Fast charge current regulation can be scaled by placing
a programming resistor (R
PROG
) from the PROG input
to V
SS
. Connecting the PROG input to V
SS
allows for a
maximum fast charge current of 400 mA, typically. The
minimum fast charge current is 85 mA (Typ) and is set
by letting the PROG input float. Equation 5-1 calculates
the value for R
PROG
.
EQUATION 5-1:
13.32
–
33.3
×
I
REG
-
R
PROG
= -----------------------------------------------
14.1
×
I
REG
–
1.2
Where:
I
REG
is the desired fast charge current in
amps
R
PROG
is in kilo-ohms.
The preconditioning trickle charge current and the
charge termination current are scaled to approximately
10% and 7% of I
REG
, respectively.
5.1.5
TIMER SET INPUT (TIMER)
The TIMER input programs the period of the safety tim-
ers 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:
C
TIMER
-
t
PRECON
= ------------------
×
1.0Hour s
0.1
µ
F
The fast charge safety timer period:
C
TIMER
-
t
FAST
= ------------------
×
1.5Hours
0.1
µ
F
And, the elapsed time termination period:
C
TIMER
-
t
TERM
= ------------------
×
3.0Hours
0.1
µ
F
The preconditioning timer starts after qualification and
resets when the charge cycle transitions to the
constant-current, fast charge phase. The fast charge
timer and the elapsed timer start after the MCP7385X
devices transition 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.
During thermal regulation, the timer is slowed down
proportional to the charge current.
DS21915A-page 13
5.1.3
CELL TEMPERATURE SENSOR
BIAS (THREF)
A 2.55V voltage reference is provided to bias an exter-
nal thermistor for continuous cell temperature monitor-
ing and prequalification. A ratio-metric window
comparison is performed at threshold levels of
V
THREF
/2 and V
THREF
/4.
5.1.4
CELL TEMPERATURE SENSOR
INPUT (THERM)
The MCP73853 continuously monitors 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
develops 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.
2004 Microchip Technology Inc.
MICROCHIP [ MICROCHIP TECHNOLOGY ]
