FPF2195 데이터 시트보기 (PDF) - Fairchild Semiconductor

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FPF2195

Full-Function Load Switch with Adjustable Current Limit

Fairchild Semiconductor 

Application Information

Figure 28. Typical Application

Setting Current Limit

The FPF2193, FPF2194, and FPF2195 current limit is

set with an external resistor connected between ISET and

GND. This resistor is selected using the following

equation:

RSET

=

551.6

ILIM

(1)

RSET is in Ω and ISET is in Amps.

Table 1 can also be used to select RSET. A typical

application would be the 500 mA current required by a

single USB port. Using Table 1, an appropriate selection

for the RSET resistor would be 788 Ω. This ensures that

the port load could draw 525 mA, but not more than

875 mA. Likewise for a dual-port system; an RSET of

368 Ω always delivers at least 1125 mA and never more

than 1875 mA.

Table 1. Current Limit Various RSET Values

RSET(Ω)

Min. Current

Limit (mA)

Typ. Current

Limit (mA)

Max. Current

Limit (mA)

368

1125

1500

1875

441

928

1250

1562

552

750

1000

1250

613

675

900

1125

690

600

800

1000

788

525

700

875

919

450

600

750

1103

375

500

625

1226

338

450

563

1379

300

400

500

1576

263

350

438

1839

225

300

375

2206

188

250

313

2758

150

200

250

3677

113

150

188

5516

75

100

125

Input Capacitor

To limit the voltage drop on the input supply caused by

transient inrush currents when the switch is turned on

into a discharged load capacitor or a short-circuit, a

capacitor needs to be placed between VIN and GND. A

0.1 µF ceramic capacitor, CIN, placed close to the pins is

usually sufficient. Higher values of CIN can be used to

further reduce the voltage drop.

Output Capacitor

A 0.1 µF capacitor, COUT, should be placed between

VOUT and GND. This capacitor prevents parasitic board

inductances from forcing VOUT below GND when the

switch turns off. For the FPF2193 and FPF2194, 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)

=

ILIM(max)× tBLANK(min)

VIN

(2)

Power Dissipation

During normal operation as a switch, the power

dissipated depends upon the level at which the current

limit is set. The maximum allowed setting for the current

limit is 1.5 A and results in a power dissipation of:

P = (ILIM)2 ×RON = (1.5)2 × 0.055 = 123.75mW (3)

If the part goes into current limit, the maximum power

dissipation occurs when the output is shorted to ground.

For the FPF2193, the power dissipation scales by the

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

time, tBLANK, so that the maximum power dissipated is:

P(max) =

tBLANK

tBLANK + tRSTRT

× VIN(max)×ILIM(max)

(4)

=

30

30 + 450

× 5.5 ×1.5

=

515.6mW

© 2009 Fairchild Semiconductor Corporation

FPF2193/4/5 • Rev. 1.0.5

11

www.fairchildsemi.com

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