MCP73871 Datasheet, PDF (21/38 Pages) Microchip Technology – Stand-Alone System Load Sharing and Li-Ion / Li-Polymer Battery Charge Management Controller

4.7 Charge Termination
The charge cycle is terminated when, during constant
voltage mode, the average charge current diminishes
below a threshold established with the value of a
resistor connected from PROG3 to V_SS or internal timer
has expired. 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:
Where:
I_TERMINATION
=
---¹---⁰---⁰---⁰---^V----
R_PROG3
R_PROG = kilo-ohms (kΩ)
I_REG = milliampere (mA)
The charge current is latched off and the MCP73871
device enters a charge complete mode. The
recommended PROG3 resistor values are between
5 kΩ and 100 kΩ.
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 once again
supplied to the battery or load. The recharge threshold
is factory set. Refer to Section 1.0 “Electrical
Characteristics” for recharge threshold options.
Note:
Charge termination and automatic
recharge features avoid constant charging
Li-Ion batteries to prolong life of Li-Ion
batteries while keeping their capacity at
healthy level.
MCP73871
4.9 Thermal Regulation
The MCP73871 device limits the charge current based
on the die temperature. The thermal regulation
optimizes the charge cycle time while maintaining
device reliability. Figure 4-3 depicts the thermal
regulation for the MCP73871 device. Refer to
Section 1.0 “Electrical Characteristics” for thermal
package resistances and Section 6.1.1.2 “Thermal
Considerations” for calculating power dissipation.
.
1200
1000
800
600
400
200
0
V_DD = 5.2V
R_PROG = 1 kΩ
25
50
75
100
125
150
Ambient Temperature (°C)
FIGURE 4-3:
Thermal Regulation
4.10 Thermal Shutdown
The MCP73871 device suspends charge if the die
temperature exceeds 150°C. Charging will resume
when the die temperature has cooled by approximately
10°C. The thermal shutdown is a secondary safety
feature in the event that there is a failure within the
thermal regulation circuitry.
4.11 Temperature Qualification
The MCP73871 device continuously monitor battery
temperature during a charge cycle by measuring the
voltage between the THERM and V_SS pins. An internal
50 µA current source provides the bias for most
common 10 kΩ negative-temperature coefficient
thermistors (NTC). The MCP73871 device compares
the voltage at the THERM pin to factory set thresholds
of 1.24V and 0.25V, typically. Once a voltage outside
the thresholds is detected during a charge cycle, the
MCP73871 device immediately suspends the charge
cycle. The MCP73871 device suspends charge by
turning off the charge pass transistor and holding the
timer value. The charge cycle resumes when the
voltage at the THERM pin returns to the normal range.
© 2009 Microchip Technology Inc.
DS22090B-page 21