FS3871 datasheet(11/13 Pages) FORTUNE

11 / 13 page

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.

C/W

)

150

(

A(MAX)

J(MAX)













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

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

Ic(max)

βmin 

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

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 the junction

temperature below the

manufacturer’s

recommended

value,

TJMAX.

Calculate the total theta,θ(°C/W), needed.

It is

recommended to choose a package with a lower

than the number calculated above.

C/W

)

150

(

A(MAX)

J(MAX)













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).

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