FPF2500 データシートの表示（PDF） - Fairchild Semiconductor

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FPF2500

IntelliMAX™ Advanced Load Management Products

Fairchild Semiconductor 

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

FPF2501, FPF2503 and FPF2505, 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)

=

I--L---I--M----(---m-----i--n---)----×-----t--B---L---A----N---K----(--m------i-n----)

VIN

(2)

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,

P = (ILIM)2 × RDS = (0.5)2 × 0.23 = 58mW

(3)

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.

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,

P = (ILIM)2 × RDS = (2)2 × 0.23 = 920mW

(4)

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,

tRESTART, and the Over Current Blanking Time, tBLANK, so that

the maximum power dissipated is,

P(max)

=

-----------------t--B---L---A---N----K------------------

tRESTART + tBLANK

×

VIN(max)

×

ILIM(max)

=

---------5----------

5 + 640

×

20

×

2.5=

388mW

(5)

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,

P(max) = VIN(max) × ILIM(max) = 20 × 2.5 = 50W (6)

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.

11

FPF2500-FPF2506 Rev. D4

www.fairchildsemi.com

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