FS3871 datasheet(10/13 Pages) FORTUNE

10 / 13 page

Condition

STAT pin

Battery conditioning and charging

High

Charge complete(done)

Low

Temperature fault or sleep mode

Hi-Z

Voltage regulation phase

The voltage regulation feedback is through the BAT

pin. This input is tied directly to the positive side of

the battery pack. The FS3871 monitors the

battery-pack voltage between the BAT and VSS

pins.

The FS3871 is offered in two fixed-voltage versions:

4.2V and 8.4 V.

Other regulation voltages can be achieved by adding

a voltage divider between the positive and negative

terminals of the battery pack .The voltage divider

presents scaled battery-pack voltage to BAT input.

The resistor values RB1 and RB2 for the voltage

divider are calculated by the following equation:

]

[

RB2

RB1

O(REG)

(CELL)





Where:

N = Number of cells in series

V(CELL) = Desired regulation voltage per cell

Automatic charge-rate compensation

For internal impedance of the battery pack, the

FS3871 uses the proprietary automatic charge-rate

compensation technique (CRC).

The CRC pin must not be left floating. The feature is

disabled by connecting the CRC pin to VCC in

high-side current-sensing configuration, and to VSS

in low-side current-sensing configuration.

Compensation is achieved through input pin CRC. A

portion of the current-sense voltage, presented

through this pin, is scaled by a factor of G(CRC) and

summed

with

the

regulation

threshold,

VO(REG).This process increases the output voltage

to compensate for the battery pack’s internal

impedance and for undesired voltage drops in the

circuit.

The CRC setup requires the following information:

1.Total impedance of battery pack (Z(PACK))

2. Maximum charging current (I(REG))

The voltage drop across the internal impedance of

battery pack, V(Z), can then be calculated using the

following equation:

]

(CRC)

O(REG)

(PACK)

(CRC)

(Z)

(CRC)





Where V(CRC) is the voltage on CRC pin. This

voltage is referenced to VCC in high-side current

sensing configuration and to VSS for low-side

sensing. V(PACK) is the voltage across the battery

pack.The values of R(CRC1) and R(CRC2) can be

calculated using the following equation:

(CRC2)

(CRC1)

(CRC2)

(SNS)

(CRC)





16.

Application Information

Selecting an external pass-transistor

The FS3871 drives an external PNP transistor or

P-channel MOSFET to control the charging current.

The specifications must be concerned are the

voltage and current rating and package power

dissipation. The following examples illustrate the

design process for either device:

PNP transistor:

FS3871 Datasheet(PDF) 10 Page - Fortune Semiconductor Corp.

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