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FPF2702 Datasheet
Adjustable Over-Current Protection Load Switches
Fairchild Semiconductor
Fairchild Semiconductor


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Input Capacitor
To limit the voltage drop on the input supply caused by
transient inrush currents when the switch is turned on
into a discharged load capacitor or short-circuit; an input
capacitor, CIN, is recommended between the IN and
GND pins. The FPF270X features a fast current limit
response time (50 μs). During this period, the device
relies on the input capacitor to supply the load current. A
10 μF to 100 μF ceramic capacitor is adequate for CIN in
most cases. Larger CIN values may be required in high-
voltage or high-current applications. An electrolytic
capacitor can be used in parallel to further reduce the
voltage drop.
Output Capacitor
A 0.1 μF to 1 μF capacitor, COUT, should be placed
between the OUT and GND pins. This capacitor helps
prevent parasitic board inductances from forcing the
output voltage below ground when the switch turns
off. This capacitor should have a low dissipation
factor. An X7R Multilayer Ceramic Chip (MLCC)
capacitor is recommended.
During startup, the total output current consists of both
the load current and the charge current of the output
capacitor. For the FPF2700 and FPF2701; if the total
output current exceeds the set current limit threshold
(determined via RSET) for longer than the blanking time,
the device may not be able to start properly. This
imposes an upper limit to the value of the output
capacitor, given the load current and the selected
current limit value. COUT should not exceed the COUTmax
calculated in Equation 2 or the switch does not start
properly due to the set current limit:
COUT max ILIM _ MIN 500s /V
(2)
Power Dissipation
During normal operation as a switch, the power
dissipation of the device is small and has little effect on
the operating temperature of the part. The maximum
power dissipation for the switch in normal operation
occurs just before the switch enters into current limit.
This may be calculated using the equation:
PD _MAX( NormalOperation) (ILIM( Max ) )2 RON( MAX )
(3)
The maximum junction temperature should be limited to
125°C under normal operation. Junction temperature
can be calculated using the equation:
TJ PD  JA TA
(4)
where:
TJ is junction temperature;
PD is power dissipation on the switch;
ΘJA is the thermal resistance, junction-to-ambient of
the package; and
TA is ambient temperature.
Design Example
For a 12 V application and ILIM (Max) = 1 A, maximum
power dissipation in a normal operation is calculated as:
PD _MAX( NormalOperation)(VIN12V ) (1)2 0.140 140mW
(5)
FPF2702 PD(Max) during OC:
If device is in over-current condition and VOUT>2 V,
power dissipation can be calculated as:
PD = (VIN - VOUT) x ILIM (Max)
(6)
High-Voltage Operation (Output Capacitor)
During a hard short condition on the output while
operating at greater than 24 V VIN, a large
instantaneous inrush current is delivered to the shorted
output. A capacitor must be placed at the OUTPUT
pin, acting as a current source to support the
instantaneous current draw (Table 2). A low-ESR
capacitor is recommended. Once the value of the
output capacitor is determined from Table 2, Equation
2 must be reevaluated.
Table 2. COUT Selection Guide
VIN (V)
Capacitance (μF)
24< VIN ≤ 27
22
27< VIN ≤ 32
47
32< VIN ≤ 36
68
If device is in short-circuit current limit and VOUT < 2 V,
power dissipation can be calculated as:
PD = (VIN - VOUT) x (0.75 x ILIM (Max) )
(7)
Design Example:
Using FPF2702 in a VIN = 5 V application where ILIM (Max)
= 2 A, assuming VOUT = 2.5 V; power dissipation across
the switch is calculated as:
PD = (5 - 2.5) x 2 = 5W
(8)
Whereas in a short-circuit current-limit condition (VOUT
0 V), power dissipation is calculated as:
PD = ((VIN - VOUT) x (0.75 x ILIM (Max)) = (5 - 0) x
(0.75 x 2) = 7.5W
(9)
© 2010 Fairchild Semiconductor Corporation
FPF2700 / FPF2701 / FPF2702 Rev. 1.0.3
14
www.fairchildsemi.com

FPF2702 Datasheet PDF



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