MCP73871 datasheet(19/36 Pages) MICROCHIP

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DS20002090F-page 19

MCP73871

4.3

Charge Qualification

For a charge cycle to begin, all UVLO conditions must

be met and a battery or output load must be present.

A charge current programming resistor must be

connected from PROG1 to VSS when SEL = high.

When SEL = low, PROG2 needs to be tied high or low

for proper operation.

4.4

Preconditioning

If the voltage at the VBAT 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 VBAT 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 VSS. 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 VBAT pin reaches the regulation voltage, VREG.

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 VBAT pin reaches the

regulation voltage, VREG, 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 VSS 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

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 VBAT 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

Section 1.0

“Electrical

Characteristics” for recharge threshold options.

IREG

1000V

RPROG1

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

Where:

RPROG

kilo-ohms (k



IREG

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.

ITERMINATION

1000V

RPROG3

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

Where:

RPROG

kilo-ohms (k



IREG

milliampere (mA)

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