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FPF2025 Datasheet

Full Functional Load Switch

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FPF2025 pdf
Application Information
Input Capacitor
To limit the voltage drop on the input supply caused by
transient in-rush currents when the switch is turned on
into a discharged load capacitor or a short-circuit, a
capacitor is recommended to be placed between VIN
and GND. The FPF2024-FPF2027 series feature a slow
turn-on to limit the inrush current and requires a smaller
input capacitor. A 1 µF ceramic capacitor, CIN, placed
close to the pins is typically sufficient. Higher values of
CIN can be used to further reduce the voltage drop.
Output Capacitor
A 0.1 µF 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 FPF2024, FPF2025 and
FPF2026, the total output capacitance needs to be kept
below a maximum value, COUT(MAX), to prevent the part
from registering an over-current condition beyond the
Blanking Time and turning-off the switch. The maximum
output capacitance can be determined from the
following formula:
(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 maximum power
dissipation while switch is in normal operation occurs
just before a part enters the current limit. This may be
calculated using the formula bellow:
(2)
If the part goes into current limit, the maximum power
dissipation occurs when the output of switch is shorted
to ground. For the FPF2024 and FPF2025, the power
dissipation will scale with the Auto-Restart Time, tRSTRT,
and the Over Current Blanking Time, tBLANK. In this case
the maximum power dissipated for the FPF2024 and
FPF2025 is:
(3)
This power dissipation is significant enough that it will
activate the thermal shutdown protection, and the part
will cycle in and out of thermal shutdown so long as the
ON pin is active and the output short is present.
Improving Thermal Performance
An improper layout could result in higher junction
temperature and ultimately trigger the thermal shutdown
protection feature. This concern applies particularly
significant for the FPF2027 where the switch is in
constant current mode in the overload conditions.
The following techniques have been identified to
improve the thermal performance of this family of
devices. These techniques are listed in order of the
significance of their impact.
NC pin can be connected to the GND plane to
improve thermal performance.
The VIN, VOUT and GND pins will dissipate most of
the heat generated during a high load current
condition. Using wide traces will help minimize
parasitic electrical effects, along with minimizing the
case to ambient thermal impedance. The layout
suggested in Figure 28 provides each pin with
adequate copper so that heat may be transferred
out of the device as efficiently as possible. The low-
power FLAGB and ON pin traces may be laid-out to
maximize the area available to the ground pad.
Placing the input and output capacitors as close to
the device as possible also contributes to heat
dissipation, particularly during high load currents.
FPF202X Demo Board
FPF202X Demo board has components and circuitry to
demonstrate the functions and features of the FPF202X
load switch family. An N-Channel MOSFET(Q), in series
with a 7.5 Ω resistor, are connected between VOUT and
the GND pin of the device. By turning on the Q
transistor, the 7.5 Ω is loaded to the output voltage and
simulates an over-current condition. The R2 resistor is
connected between FLAGB pin and input voltage as
pull-up resistor for FPF2024, FPF2026 and FPF2027
devices. The FPF2025 does not require a pull-up
resistor due to its CMOS output structure.
The thermal performance of the board is improved using
the techniques recommended in the layout
recommendations section of datasheet.
Take note that this is below the maximum package
power dissipation, and the thermal shutdown feature
protection provides additional safety to protect the part
from damage due to excessive heating. The junction
temperature is only able to increase to the thermal
shutdown threshold. Once this temperature has been
reached, toggling ON will have no effect until the
junction temperature drops below the thermal shutdown
exit temperature. For the FPF2027, a short on the
output will cause the part to operate in a constant
current state dissipating a worst case power of:
(4)
Output of FPF2027 is shorted to GND.
© 2008 Fairchild Semiconductor Corporation
FPF2024/25/26/27 Rev. 1.1
14
Figure 28. FPF2020X Proper Layout
www.fairchildsemi.com
Fairchild Semiconductor
Fairchild Semiconductor


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