fs3871 Fortune Semiconductor Corporation, fs3871 Datasheet - Page 11

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

Rev. 1.2
Selection steps for a PNP bipolar transistor:
Example: VI = 4.5 V, I(REG) = 1 A, 4.2-V single-cell
Li-Ion .VI is the input voltage to the charger and
I(REG) is the desired charge current (see Figure 1).
1. Determine the maximum power dissipation, PD, in
the transistor.
The worst case power dissipation happens when the
cell voltage, V(BAT), is at its lowest (typically 3 V at
the beginning of current regulation phase) and VI is
at its maximum. Where VCS is the voltage drop
across the current sense resistor.
PD=(VI－VCS－VBAT)×IREG
PD=(4.5－0.1－3.)×1A
PD=1.4W
2. Determine the package size needed in order
to keep the junction temperature below the
manufacturer’s recommended value, T(J)max.
Calculate the total theta, ( C/W), needed.
Now choose a device package with a theta at least
10% below this value to account for additional thetas
other than the device. A SOT223 package, for
instance, has typically a theta of 60 C/W.
3. Select a collector-emitter voltage, V(CE), rating
greater than the maximum input voltage. A 15-V
device will be adequate in this example. Select a
device that has at least 50% higher drain current IC
rating than the desired charge current I(REG).
4. Using the following equation, calculate the
minimum beta (β o r hFE) needed:
where Ic(max) is the maximum collector current (in
this case same as I(REG)), and IB is the base
current.
Selecting input/output capacitor
In analog circuit applications, all that is needed is a
high-frequency decoupling capacitor. A 0.1 uF
ceramic, placed in proximity to VCC and VSS pins,
works well. If a high ripple and noise input power is
chosen, it should have enough capacitance to
reduce the disturbance. A 0.1uF to 10uF output
capacitor is recommended to control the output
voltage and keep the output voltage ripple small
when the battery is disconnected.
P-Channel MOSFET
βmin =
θ
θ
θ
JA
JA
JA
=
=
=
78
T
(
150
J(MAX)
°
Ic(max)
1
C/W
4 .
−
I
B
40
P
−
D
T
)
A(MAX)
Selection steps for a P-channel MOSFET: We will
use the following conditions: VI=5V; I(REG)=1A,
4.2-V single-cell Li-Ion. VI is the input voltage to the
charger and I(REG) is the desired charge current.
1. Determine the maximum power dissipation, PD ,
in the transistor.The worst case power dissipation
happens when the cell voltage, V(constant), is at its
lowest (typically 3.1V at the beginning of current
regulation phase) and VI is at its maximum. Where
VD is the forward voltage drop across the
reverse-blocking diode (if one is used), and VCS is
the voltage drop across the current sense resistor.
PD=(VI(MAX)－VD－VCS－VBAT)×IREG
PD=(5.5－0.4－0.2－3.1)×1A
PD=1.8W
2. Determine the package size needed in order to
keep
manufacturer’s
Calculate the total theta,θ(°C/W), needed. It is
recommended to choose a package with a lower
3. Select a drain-source voltage, V(DS), rating
greater than the maximum input voltage. A 12V
device will be adequate in this example.
4. Select a device that has at least 50% higher drain
current (ID) rating than the desired charge current
I(REG).
5. Verify that the available drive is large enough to
supply the desired charge current.
V(GS)=(VD+V(CS)+VOL(CC))－VI(min)
V(GS)=(0.4+0.2+1)
V(GS)=-2.9
Where V(GS) is the gate-to-source voltage, VD is
the forward voltage drop across the reverse-blocking
diode.(if one is used), and VCS is the voltage drop
across the current sense resistor, and VOL(CC) is
the CC pin output low voltage specification for the
FS3871.
Select a MOSFET with gate threshold voltage,
V(GSth),rating less than the calculated V(GS).
θ
JA
than the number calculated above.
the
θ
θ
θ
junction
JA
JA
JA
=
=
=
recommended
61
T
(
150
J(MAX)
°
C/W
1
temperature
8 .
−
40
P
−
D
T
)
A(MAX)
value,
below
FS3871
TJMAX.
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the