Datasheet
2008-2013 Microchip Technology Inc. DS20002090C-page 23
MCP73871
4.4 Preconditioning
If the voltage at the V
BAT
pin is less than the
preconditioning threshold, the MCP73871 device
enters a preconditioning mode. The preconditioning
threshold is factory set. Refer to Section 1.0
“Electrical Characteristics” for preconditioning
threshold options.
In this mode, the MCP73871 device supplies 10% of
the fast charge current (established with the value of
the resistor connected to the PROG1 pin) to the
battery.
When the voltage at the V
BAT
pin rises above the
preconditioning threshold, the MCP73871 device
enters the Constant Current (fast charge) mode.
4.5 Constant Current Mode – Fast
Charge
During the Constant Current mode, the programmed
charge current is supplied to the battery or load. The
charge current is established using a single resistor
from PROG1 to V
SS
. The program resistor and the
charge current are calculated using the following
equation:
EQUATION 4-1:
Constant Current mode is maintained until the voltage
at the V
BAT
pin reaches the regulation voltage, V
REG
.
When Constant Current mode is invoked, the internal
timer is reset.
4.5.1 TIMER EXPIRED DURING
CONSTANT CURRENT - FAST
CHARGE MODE
If the internal timer expires before the recharge voltage
threshold is reached, a timer fault is indicated and the
charge cycle terminates. The MCP73871 device
remains in this condition until the battery is removed. If
the battery is removed, the MCP73871 device enters
the Standby mode where it remains until a battery is
reinserted.
4.6 Constant Voltage Mode
When the voltage at the V
BAT
pin reaches the
regulation voltage, V
REG
, constant voltage regulation
begins. The regulation voltage is factory set to 4.10V
or 4.20V with a tolerance of ±0.5%.
4.7 Charge Termination
The Constant Voltage mode charge cycle terminates
either when the average charge current diminishes
below a threshold established by the value of the
resistor connected from PROG3 to V
SS
or when the
internal charge timer expires. When the charge cycle
terminates due to a fully charged battery, the charge
current is latched off and the MCP73871 device enters
the Charge Complete mode. A 1 ms filter time on the
termination comparator ensures that transient load
conditions do not result in premature charge cycle
termination. The timer period is factory set and can be
disabled. Refer to Section 1.0 “Electrical
Characteristics” for timer period options.
The program resistor and the charge current are
calculated using the following equation:
EQUATION 4-2:
The recommended PROG3 resistor values are
between 5 k and 100 k.
4.8 Automatic Recharge
The MCP73871 device continuously monitors the
voltage at the V
BAT
pin in the charge complete mode. If
the voltage drops below the recharge threshold,
another charge cycle begins and current is supplied
again to the battery or load. The recharge threshold is
factory set. Refer to Section 1.0 “Electrical
Characteristics” for recharge threshold options.
I
REG
1000V
R
PROG1
--------------------=
Where:
R
PROG
= kilo-ohms (k
I
REG
= milliampere (mA)
Note: Charge termination and automatic
recharge features avoid constantly
charging Li-Ion batteries, resulting in
prolonged battery life while maintaining
full cell capacity.
I
TERMINATION
1000V
R
PROG3
--------------------=
Where:
R
PROG
= kilo-ohms (k
I
REG
= milliampere (mA)