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FPF1038 Datasheet
Slew-Rate-Controlled Load Switch
Fairchild Semiconductor
Fairchild Semiconductor


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FPF1038 pdf
Application Information
Input Capacitor
This IntelliMAX™ switch doesn’t require an input
capacitor. To reduce device inrush current, a 0.1 µF
ceramic capacitor, CIN, is recommended close to the
VIN pin. A higher value of CIN can be used to reduce the
voltage drop experienced as the switch is turned on into
a large capacitive load.
Output Capacitor
While this switch works without an output capacitor: if
parasitic board inductance forces VOUT below GND
when switching off; a 0.1 µF capacitor, COUT, should be
placed between VOUT and GND.
Fall Time
Device output fall time can be calculated based on RC
constant of the external components as follows:
tF = RL × CL × 2.2
(1)
where tF is 90% to 10% fall time, RL is output load,
and CL is output capacitor.
The same equation works for a device with a pull-down
output resistor. RL is replaced by a parallel connected
pull-down and an external output resistor combination as:
tF
=
RL
RL
× RPD
+ RPD
× CL
× 2.2
(2)
where tF is 90% to 10% fall time, RL is output load,
RPD=65 is output pull-down resistor, and CL is the
output capacitor.
Resistive Output Load
If resistive output load is missing, the IntelliMAX switch
without a pull-down output resistor does not discharge
the output voltage. Output voltage drop depends, in that
case, mainly on external device leaks.
Application Specifics
At maximum operational voltage (VIN=5.5 V), device
inrush current might be higher than expected. Spike
current should be taken into account if VIN>5 V and the
output capacitor is much larger than the input capacitor.
Input current can be calculated as:
IIN(t)
VOUT (t)
RLOAD
+ (CLOAD
CIN
)
dVOUT
dt
(t
)
(3)
where switch and wire resistances are neglected and
capacitors are assumed ideal.
Estimating VOUT(t)=VIN/10 and using experimental
formula for slew rate (dVOUT(t)/dt), spike current can be
written as:
max(IIN) =
VIN
10RLOAD
+ (CLOAD
CIN )0.05VIN
0.255
(4)
where supply voltage VIN is in volts, capacitances are
in micro farads, and resistance is in ohms.
Example: If VIN=5.5 V, CLOAD=100 µF, CIN=10 µF, and
RLOAD=50 ; calculate the spike current by:
max(IIN )
=
5.5
1050
+
(100
10)(0.055.5
0.255)A
=
1.8A
(5)
Maximum spike current is 1.8 A, while average ramp-
up current is:
IIN(t)
VOUT (t )
RLOAD
+
(CLOAD
CIN
)
dVIN(t)
dt
2.75 / 50 + 1000.0022 = 0.275A
(6)
Recommended Layout
For best thermal performance and minimal inductance
and parasitic effects, it is recommended to keep input
and output traces short and capacitors as close to the
device as possible. Figure 29 is a recommended layout
for this device to achieve optimum performance.
Vin Vout
VIN FPF1038
CIN OFF ON ON
GND
+
CLOAD
RLOAD
Figure 28. Device Setup
Figure 29. Recommended Land Pattern, Layout
© 2010 Fairchild Semiconductor Corporation
FPF1038 • Rev. 1.0.6
10
www.fairchildsemi.com

FPF1038 Datasheet PDF



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