fs3871 Fortune Semiconductor Corporation, fs3871 Datasheet - Page 9

• Manufacturer Part Number: fs3871
• Description: Linear Charge Management Ic For Lithium-ion And Lithium-polymer
• Manufacturer: Fortune Semiconductor Corporation

Rev. 1.2
14. 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:
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
threshold.(See figure 2)
Battery temperature monitoring
The FS3871 continuously monitors temperature by
measuring the voltage between the TS and VSS
pins.
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.)
A
voltage
negative-
R
SNS
falls
or
=
Io
V
a
below
(
(sns)
REG)
positive-temperature
the
V(RCH)
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 .
Figure3 TS Input Thresholds
FS3871
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