MCP73871 datasheet(21/38 Pages) MICROCHIP

21 / 38 page

DS22090B-page 21

MCP73871

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 VSS 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:

The charge current is latched off and the MCP73871

device

enters

charge

complete

mode.

The

recommended PROG3 resistor values are between

Ω and 100 kΩ.

4.8

Automatic Recharge

The MCP73871 device continuously monitors the

voltage at the VBAT 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.

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

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 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 VSS pins. An internal

50 µA current source provides the bias for most

common

10 k

Ω 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.

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.

TERMINATION

1000V

PROG3

--------------------

Where:

RPROG

kilo-ohms (k

Ω)

IREG

milliampere (mA)

200

400

600

800

1000

1200

100

125

150

Ambient Temperature (°C)

VDD = 5.2V

RPROG = 1 kΩ

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