FPF2500_07 FAIRCHILD [Fairchild Semiconductor], FPF2500_07 Datasheet - Page 11

• Manufacturer Part Number: FPF2500_07
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

FPF2500-FPF2506 Rev. D4
P max
Output Capacitor
A 0.1uF capacitor C
GND. This capacitor will prevent parasitic board inductances
from forcing V
FPF2501, FPF2503 and FPF2505, the total output capacitance
needs to be kept below a maximum value, C
the part from registering an over-current condition and turning
off the switch. The maximum output capacitance can be
determined from the following formula,
Power Dissipation
Due to the wide input voltage range that the parts can accept
and the high currents at which the current limit can be set, care
must be taken to prevent excessive power dissipation in the part
from interrupting the switching of power to a load. During normal
operation as a switch, the power dissipated in the part will
depend upon the level at which the current limit is set. For a
0.5A typical load, the power dissipated will typically be,
This is well within the thermal capability of the package. The
maximum allowed setting for the current limit is 2A and this will
result in a power dissipation of typically,
This is beyond the normal thermal capability of the package, but
the thermal shutdown within the part will prevent damage by
turning off the switch once the maximum junction temperature
has been reached. If the part goes into current limit the
maximum power dissipation will occur when the output is
shorted to ground. For the FPF2500, FPF2503 and FPF2505
the power dissipation will scale by the Auto-Restart Time,
t
the maximum power dissipated is,
This is more power than the package can dissipate, but the
thermal shutdown of the part will activate to protect the part
from damage due to excessive heating. The junction
temperature will only be able 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 FPF2502, FPF2504 and FPF2506,
a short on the output will cause the part to operate in a constant
current state dissipating a worst case power of,
This large amount of power will activate the thermal shutdown
and the part will cycle in and out of thermal shutdown so long as
the ON pin is active and the short is present.
P max
RESTART
(
(
P
P
C
=
)
=
)
OUT
, and the Over Current Blanking Time, t
=
(
=
=
(
I
I
LIM
LIM
V
--------------------------------------------------
t
(
------------------ -
5
OUT
RESTART
max
IN
+
)
)
(
5
2
2
640
max
below GND when the switch turns-off. For the
×
×
)
t
OUT,
R
BLANK
R
=
×
DS
) I
DS
20
×
I
----------------------------------------------------------------- -
+
LIM
should be placed between V
=
t
=
LIM
BLANK
×
(
(
2 ( )
2.5
min
0.5
(
max
2
=
)
) t
2
×
×
×
388mW
V
×
0.23
)
V
IN
BLANK
0.23
IN
=
(
20 2.5
max
=
OUT(max)
=
×
920mW
(
min
58mW
)
BLANK
×
I
)
, to prevent
LIM
=
OUT
, so that
50W
(
max
and
(2)
(3)
(4)
(5)
(6)
)
11
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 V
minimize parasitic electrical effects along with minimizing the
case to ambient thermal impedance.
IN
, V
OUT
and GND will help
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