MCP73871 datasheet(21/36 Pages) MICROCHIP

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DS20002090F-page 21

MCP73871

5.0

DETAILED DESCRIPTION

5.1

Analog Circuitry

5.1.1

LOAD SHARING AND LI-ION

BATTERY MANAGEMENT INPUT

SUPPLY (VIN)

The VIN input is the input supply to the MCP73871

device. The MCP73871 device can be supplied by

either AC Adapter (VAC) or USB Port (VUSB) with SEL

pin. The MCP73871 device automatically powers the

system with the Li-Ion battery when the VIN input is not

present.

5.1.2

FAST CHARGE CURRENT

REGULATION SET (PROG1)

For the MCP73871 device, the charge current

regulation can be scaled by placing a programming

resistor (RPROG1) from the PROG1 pin to VSS. The

program resistor and the charge current are calculated

using the following equation:

EQUATION 5-1:

The fast charge current is set for maximum charge

current from AC-DC adapter and USB port. The

preconditioning current is 10% (0.1C) of the fast charge

current.

5.1.3

BATTERY CHARGE CONTROL

OUTPUT (VBAT)

The battery charge control output is the drain terminal

of an internal P-channel MOSFET. The MCP73871

device

provides constant current and voltage

regulation to the battery pack by controlling this

MOSFET in the linear region. The battery charge

control output should be connected to the positive

terminal of the battery pack.

5.1.4

TEMPERATURE QUALIFICATION

(THERM)

The MCP73871 device continuously monitors 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

 NTC or Positive Temperature

Coefficient (PTC) thermistors.The current source is

controlled, avoiding measurement sensitivity to

fluctuations in the supply voltage (VDD). 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 the charge by turning

off the pass transistor and holding the timer value. The

charge cycle resumes when the voltage at the THERM

pin returns to the normal range.

If temperature monitoring is not required, place a

standard 10 k

 resistor from THERM to VSS.

5.2

Digital Circuitry

5.2.1

STATUS INDICATORS AND POWER

GOOD (PG)

The charge status outputs have two different states:

Low-Impedance (L) and High-Impedance (High-Z).

The charge status outputs can be used to illuminate

LEDs. Optionally, the charge status outputs can be

used as an interface to a host microcontroller. Table 5-

1 summarizes the state of the status outputs during a

charge cycle.

TABLE 5-1:

STATUS OUTPUTS

IREG

1000V

RPROG1

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

Where:

RPROG

kilo-ohms (k



IREG

milliampere (mA)

CHARGE CYCLE STATE

STAT1

STAT2

Shutdown (VDD =VBAT)

High-Z

Shutdown (VDD = IN)

High-Z

Shutdown (CE = L)

High-Z

Preconditioning

High-Z

Constant Current

High-Z

Constant Voltage

High-Z

Charge Complete - Standby

High-Z

Temperature Fault

Timer Fault

Low Battery Output

High-Z

No Battery Present

High-Z

No Input Power Present

High-Z

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