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Fabricante

FPF2300MPX

Dual-Output Current Limit Switch

Fairchild Semiconductor 

Host

5V

1μF

OFF ON

OFF ON

10KΩ

10KΩ

FLAGB(A)

IN

FLAGB(B)

FPF2300/2/3

ONA

OUTA

ONB

GND OUTB

33μF

Downstream

USB Port

Figure 40. Self-Powered USB Port for High Current Demand

High current, over 2A, is sometimes required to supply enough

power to downstream functions. As shown in Figure 40, a 2.2A

minimum load current can be achieved by tying dual outputs

together.

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

both switches while the switch is in normal operation occurs just

before both channels enter into current limit. This may be

calculated using the formula:

PD_MAX(Normal Operation) = 2 x (ILIM(MIN))2 x RON(MAX)

(3)

For example, for a 5V application, maximum normal operation

power loss while both switches delivering output current up to

1.1A, can be calculated as:

PD_MAX(Normal Operation)(IN = 5V) = 2 x (1.1)2 x 0.14

(4)

= 338mW

The maximum junction temperature should be limited to 125°C

under normal operation. Junction temperature can be calcu-

lated using the formula below:

TJ = PD x RθJA + TA

(5)

where:

TJ is junction temperature;

PD is power dissipation across the switch;

RθJA is thermal resistance junction to ambient of the package;

TA is ambient temperature.

For the example, TJ(MAX)(Normal operation) for an SO8 package

with TA=25°C while both switches are delivering up to 1.1A is

calculated as:

TJ(MAX)(NormalOperation)

= PD_MAX(Normal Operation)(IN = 5V) x 125 + 25

(6)

= 78.4°C

If the part goes into current limit, the maximum power

dissipation occurs when the output of switch is shorted to

ground. For the FPF2300 the power dissipation scales with the

auto-restart time, tRSTRT, and the over-current blanking time,

tBLANK. In this case, the maximum power dissipated for the

FPF2300 is::

tBLANK

PD_MAX(CurrentLimit) = 2 x tBLANK + tRSTRT x IN(MAX) x ILIM(MAX)

(7)

which results in:

10

PD_MAX(CurrentLimit) = 2 x 10 + 504 x 5.5 x 1.5 = 321mW (8)

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 has no affect until the junction

temperature drops below the thermal shutdown exit

temperature. For the FPF2303, a short on both outputs causes

both switches to operate in a constant current state and

dissipate a worst-case power of:

PMAX = 2 x IN(MAX) x ILIM(MAX) = 2 x 5.5 x1.5 = 16.5 W (9)

As both FPF2303 outputs are connected to GND.

This power dissipation is significant and activates both thermal

shutdown blocks and the part can cycle in and out of thermal

shutdown as long as the ON pin is activated (pulled LOW) and

the output short is present.

© 2009 Fairchild Semiconductor Corporation

FPF2300/02/03 • Rev. 1.1.3

14

www.fairchildsemi.com

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