fs3871 Fortune Semiconductor Corporation, fs3871 Datasheet - Page 10

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

Rev. 1.2
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:
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
Condition
Battery conditioning and charging
Charge complete(done)
Temperature fault or sleep mode
RB2
RB1
=
N [
*
V
V
(CELL)
O(REG)
]
−
1
STAT pin
High
Low
Hi-Z
through this pin, is scaled by a factor of G(CRC) and
summed
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.
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:
V(Z) = Z(PACK)
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:
15. 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:
V
V
V
V
The CRC setup requires the following information:
(CRC)
(PACK)
(CRC)
(SNS)
=
=
=
G
R
V
V
with
(CRC)
O(REG)
(CRC1)
(Z)
R
 I (REG)
(CRC2)
+
+
the
R
[G
(CRC2)
(CRC)
regulation
*
V
(CRC)
]
threshold,
FS3871
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