Datasheet

MCP73871
DS20002090C-page 24  2008-2013 Microchip Technology Inc.
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.
.
FIGURE 4-3: Thermal Regulation.
4.10 Thermal Shutdown
The MCP73871 device suspends charge if the die
temperature exceeds 150°C. Charging resumes 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 monitors 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 k NTC thermistors. 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 charging 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.
4.12 Voltage Proportional Charge
Control (VPCC)
If the voltage on the IN pin drops to a preset value,
determined by the threshold established at the VPCC
input, due to a limited amount of input current or input
source impedance, the battery charging current is
reduced. The VPCC control tries to reach a steady
state condition where the system load has priority and
the battery is charged with the remaining current.
Therefore, if the system demands more current than
the input can provide, the ideal diode becomes
forward-biased and the battery may supplement the
input current to the system load.
The VPCC sustains the system load as its highest
priority. It does this by reducing the noncritical charge
current while maintaining the maximum power output of
the adapter. Further demand from the system is
supported by the battery, if possible.
The VPCC feature functions identically for USB port or
AC-DC adapter inputs. This feature can be disabled by
connecting the VPCC to IN pin.
4.13 Input Current Limit Control (ICLC)
If the input current threshold is reached, then the
battery charging current is reduced. The ICLC tries to
reach a steady state condition where the system load
has priority and the battery is charged with the
remaining current. No active control limits the current
to the system. Therefore, if the system demands more
current than the input can provide or the ICLC is
reached, the ideal diode becomes forward biased and
the battery may supplement the input current to the
system load.
The ICLC sustains the system load as its highest
priority. This is done by reducing the non-critical charge
current while adhering to the current limits governed by
the USB specification or the maximum AC-DC adapter
current supported. Further demand from the system is
supported by the battery, if possible.
FIGURE 4-4: Input Current Limit Control -
USB Port.