FS3871 datasheet(9/13 Pages) FORTUNE

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15.

Detail Description

Precharge mode

FS3871’s charge qualification is based on battery

temperature and voltage. The FS3871 suspends

charge if the battery temperature is outside the

V(TS1) to V(TS2) range and suspends charge until

the battery temperature is within the allowed range.

The FS3871 also checks the battery voltage. If the

battery voltage is below the precharge threshold

V(min), the FS3871 uses precharge to condition the

battery. The conditioning charge rate I(PRECHG) is

set at approximately 10% of the regulation current.

The conditioning current also minimizes heat

dissipation in the external pass-element during the

initial stage of charge. See Figure 2.

Sleep mode

The FS3871 enters the sleep mode if the VCC falls

below the voltage at the BAT input. This feature

prevents draining the battery pack during the

absence of VCC.

Constant current regulation charge mode

The FS3871 regulates current while the battery-pack

voltage is less than the regulation voltage, VO(REG).

The FS3871 monitors charge current at the SNS

input by the voltage drop across a sense-resistor,

RSNS, in series with the battery pack. In high-side

current sensing configuration (Figure 1), RSNS is

between the VCC and SNS

The following formula calculates the value of the

sense resistor:

REG)

(sns)

SNS

(



Where IO(REG) is the desired charging current.

Charge termination and recharge

The FS3871 monitors the charging current during

the voltage-regulation phase. The FS3871 declares

a done condition and terminates charge when the

current tapers off to the charge termination threshold,

I(TERM). A new charge cycle begins when the

battery

voltage

falls

below

the

V(RCH)

threshold.(See figure 2)

Battery temperature monitoring

The FS3871 continuously monitors temperature by

measuring the voltage between the TS and VSS

pins. A negative- or a positive-temperature

coefficient thermistor (NTC, PTC) and an external

voltage divider typically develop this voltage. (See

Figure 1.) The FS3871 compares this voltage

against its internal V(TS1) and V(TS2) thresholds to

determine if charging is allowed. (See Figure 3.)

Figure3 TS Input Thresholds

The temperature sensing circuit is immune to any

fluctuation in VCC, since both the external voltage

divider and the internal thresholds (V(TS1) and

V(TS2)) are referenced to VCC.The resistor values

of R(T1) and R(T2) are calculated by the following

equations:

For NTC Thermistors:

For PTC Thermistors

Where R(TC) is the cold temperature resistance and

R(TH) is the hot temperature resistance of thermistor,

as specified by the thermistor manufacturer.RT1 or

RT2 can be omitted If only one temperature (hot or

cold) setting is required. Applying a voltage between

the V(TS1) and V(TS2) thresholds to pin TS disables

the temperature-sensing feature.

Charge status indication

The FS3871 indicates the status of the charger on

the 3-state STAT pin. The following table shows the

statuses of this pin .

FS3871 Datasheet(PDF) 9 Page - Fortune Semiconductor Corp.

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