MCP73871-2CCISLASHML datasheet(23/38 Pages) MICROCHIP

23 / 38 page

DS22090B-page 23

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) to 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

Ω negative-temperature coefficient

(NTC)

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 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

Ω resistor from THERM to V

SS.

5.2

Digital Circuitry

5.2.1

STATUS INDICATORS AND

POWER-GOOD (PG)

The charge status outputs have two different states:

Low (L), and High Impedance (Hi-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

5.2.2

AC-DC ADAPTER AND USB PORT

POWER SOURCE REGULATION

SELECT (SEL)

With the SEL input Low, the MCP73871 device is

designed to provide system power and Li-Ion battery

charging from a USB input while adhering to the current

limits governed by the USB specification. The host

microcontroller has the option selecting either a

100 mA (L) or a 500 mA (H) current limit based on the

PROG2 input. With the SEL input High, the MCP73871

device

limits

the

input

current

1.8A.

The

programmed charge current is established using a

single resistor from PROG1 to VSS when driving SEL

High.

REG

1000V

PROG1

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

Where:

RPROG

kilo-ohms (k

Ω)

IREG

milliampere (mA)

CHARGE CYCLE STATE

STAT1

STAT2

Shutdown (VDD = VBAT)

Hi-ZHi-ZHi-Z

Shutdown (VDD = IN)

Hi-Z

Preconditioning

Hi-Z

Constant Current

Hi-Z

Constant Voltage

Hi-Z

Charge Complete - Standby

Hi-Z

Temperature Fault

Timer Fault

Low Battery Output

Hi-Z

No Battery Present

Hi-Z

No Input Power Present

Hi-Z

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