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FPF2140 Datasheet

Full Function Load Switch

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FPF2140 pdf
Application Information
Typical Application
Battery
1.8V-5.5V
Typical value = 100K
OFF ON
C1 = 4.7µF
VIN VOUT
FPF2140/2/3/4/6/7
PGOOD
ON
GND
FLAGB
LOAD
R1 = 100K
C2 = 0.1µF
R2 = 499
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 4.7µF ceramic capacitor, CIN, must be
placed close to the VIN pin. A higher value of CIN can be used to
further reduce the voltage drop experienced as the switch is
turned on into a large capacitive load.
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
30 + 450
x 5.5 x 0.8 = 275mW
(3)
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.
FPF2140/42/43/44/46/47 Rev. I
10
www.fairchildsemi.com
Fairchild Semiconductor
Fairchild Semiconductor



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