FPF2140 查看數據表（PDF） - Fairchild Semiconductor

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FPF2140

Full Function Load Switch with Reverse Current Blocking

Fairchild Semiconductor 

Application Information

Typical Application

Battery

1.8V-5.5V

Typical value = 100KΩ

OFF ON

C1 = 0.1µF

VIN

VOUT

FPF2140/2/3/4/6/7

PGOOD

ON

GND

FLAGB

LOAD

R1 = 100KΩ

C2 = 0.1µF

R2 = 499Ω

Input Capacitor

To limit the voltage drop on the input supply caused by transient

in-rush currents when the switch turns-on into a discharged load

capacitor or a short-circuit, a capacitor needs to be placed

between VIN and GND. A 0.1µF ceramic capacitor, CIN, placed

close to the pins is usually sufficient. Higher values of CIN can

be used to further reduce the voltage drop.

Output Capacitor

A 0.1uF capacitor COUT, should be placed between VOUT and

GND. This capacitor will prevent parasitic board inductances

from forcing VOUT below GND when the switch turns-off. For the

FPF2140/42/44/46, the total output capacitance needs to be

kept below a maximum value, COUT(max), to prevent the part

from registering an over-current condition and turning-off the

switch. The maximum output capacitance can be determined

from the following formula,

COUT(max) =

ILIM(max) x tBLANK(min)

VIN

(1)

Power Dissipation

During normal operation as a switch, the power dissipation is

small and has little effect on the operating temperature of the

part. The parts with the higher current limits will dissipate the

most power and that will only be,

P = (ILIM)2 x RDS = (0.8)2 x 0.12 = 76.8mW

(2)

If the part goes into current limit the maximum power dissipation

will occur when the output is shorted to ground. For the

FPF2140/44, the power dissipation will scale by the

Auto-Restart Time, tRSTRT, and the Over Current Blanking Time,

tBLANK, so that the maximum power dissipated is,

P(max) =

tBLANK

tBLANK + tRSTRT

x VIN(max) x ILIM(max)

30

=

x 5.5 x 0.8 = 275mW

(3)

30 + 450

When using the FPF2142/46 attention must be given to the

manual resetting of the part. The junction temperature will only

be allowed to increase to the thermal shutdown threshold. Once

this temperature has been reached, toggling ON will not turn-on

the switch until the junction temperature drops. For the

FPF2140/44, a short on the output will cause the part to operate

in a constant current state dissipating a worst case power as

calculated in (3) until the thermal shutdown activates. It will then

cycle in and out of thermal shutdown so long as the ON pin is

active and the short is present.

Board Layout

For best performance, all traces should be as short as possible.

To be most effective, the input and output capacitors should be

placed close to the device to minimize the effects that parasitic

trace inductances may have on normal and short-circuit

operation. Using wide traces for VIN, VOUT and GND will help

minimize parasitic electrical effects along with minimizing the

case to ambient thermal impedance.

The middle pad (pin 7) should be connected to the GND plate

of PCB for improving thermal performance of the load switch.

An improper layout could result higher junction temperature and

triggering the thermal shutdown protection feature. This concern

applies specially with FPF2143 and FPF2147 where load switch

turns on into an overcurrent condition and switch supplies

constant current limit value. In this case power dissipation of the

switch (PD = (VIN - VOUT) x ILIM(max)) could exceed the

maximum absolute power dissipation of 1.2W.

10

FPF2140/42/43/44/46/47 Rev. D

www.fairchildsemi.com

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