FPF2123 datasheet(11/15 Pages) ONSEMI

11 / 15 page

www.fairchildsemi.com

FPF2123-FPF2125 Rev. F

Application Information

Typical Application

OFF ON

VIN

GND

FPF2123- FPF2125

ISET

VOUT

Battery

5.5V

C1=4.7 F

RSET

C2=0.1 F

R2=110

5.5V MAX

Setting Current Limit

The FPF2123, FPF2124, and FPF2125 have a current limit

which is set with an external resistor connected between ISET

and GND. This resistor is selected by using the following

equation,

RSET is in Ohms and that of ILIM is Amps

The table below can also be used to select RSET. A typical

application would be the 500mA current that is required by a

single USB port. Using the table below an appropriate selection

for the RSET resistor would be 604 . This will ensure that the

port load could draw 570mA, but not more than 950mA.

Likewise for a dual port system, an RSET of 340 would always

deliver at least 1120mA and never more than 1860mA.

Input Capacitor

To limit the voltage drop on the input supply caused by transient

in-rush currents when the switch turns-on into a discharged load

capacitance or a short-circuit, a capacitor needs to be placed

between VIN and GND. A 4.7 F ceramic capacitor, CIN, must be

placed close to the VIN pin. A higher value of CIN can be used to

further reduce the voltage drop experienced as the switch is

turned on into a large capacitive load.

Output Capacitor

A 0.1 F 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

FPF2123 and FPF2124, 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,

Current Limit Various RSET Values

Power Dissipation

During normal operation as a switch, the power dissipated in the

part will depend upon the level at which the current limit is set.

The maximum allowed setting for the current limit is 1.5A and

this will result in a typical power dissipation of,

If the part goes into current limit the maximum power dissipation

will occur when the output is shorted to ground. For the

FPF2123 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,

SET

460

LIM

----------

OU T m a x

(

)

LIM m i n

(

)

BLANK m i n

(

)

-----------------------------------------------------------------

RSET

[ ]

Min. Current

Limit

[mA]

Typ. Current

Limit

[mA]

Max. Current

Limit

[mA]

309

1120

1490

1860

340

1010

1350

1690

374

920

1230

1540

412

840

1120

1400

453

760

1010

1270

499

690

920

1150

549

630

840

1050

576

600

800

1000

604

570

760

950

732

470

630

790

887

390

520

650

1070

320

430

540

1300

260

350

440

1910

180

240

300

3090

110

150

190

LIM

(

)

1.5

(

)

0.125

281mW

(4)

-------------------

5.5

1.5

1.65W

P max

(

)

BL ANK m a x

(

)

REST ART m i n

(

)

BLANK m a x

(

)

---------------------------------------------------------------------------------

IN m a x

(

)

LIM m ax

(

)

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