10KΩ

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10KΩ

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Host

 

 

 

 

 

 

 

IN

 

 

 

 

 

 

FLAGB(A)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5V

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

FLAGB(B)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

FPF2300/2/3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1μF

 

 

 

OFF

ON

 

 

ONA

 

 

 

 

 

 

 

OUTA

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

OFF

ON

 

 

ONB

GND

 

OUTB

 

 

 

 

 

 

 

 

 

Downstream

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

USB Port

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

33μF

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

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

minimum load current can be achieved by tying dual outputs

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

together.

Switch Limit Current Output-Dual — FPF2300/02/03

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:

P

D_MAX(Normal Operation)

= 2 x (I

)2 x R

ON(MAX)

(3)

 

LIM(MIN)

 

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 calculated 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:

 

 

 

PD_MAX(CurrentLimit) = 2 x

10

x 5.5 x 1.5 = 321mW

(8)

 

10 + 504

 

 

 

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

www.fairchildsemi.com

FPF2300/02/03 • Rev. 1.1.3

14