LTC4241

14

sn4241 4241f

Table 4. N-Channel Power MOSFET Selection Guide

CURRENT

PART NUMBER

PACKAGE

MANUFACTURER

RATING

8.0A

Si4412ADY

SO-8

30V

±20V

0.024

Ω

Vishay-Siliconix

3.5A

Si2306DS

SOT-23

30V

±20V

0.057

Ω

Vishay-Siliconix

10A

Si4410DY

SO-8

30V

±20V

0.013

Ω

Vishay-Siliconix

13A

IRF7413

SO-8

30V

±20V

0.011

Ω

International Rectifier

2.7A

FDN 359AN

SOT-23

30V

±20V

0.046

Ω

Fairchild Semiconductor

Electronic Circuit Breaker for 3.3V Auxiliary Supply

An electronic circuit breaker is used to protect against

excessive load current and short-circuits on the 3.3V

auxiliary supply. The load current is monitored by placing

a sense resistor R3 between AUXIN and AUXSENSE as

shown in Figure 1. The circuit breaker trip threshold is

50mV and exhibits a response time of 8

µs. Unlike the PCI

supplies which use the current foldback limit with circuit

breaker during short-circuits, here the circuit breaker will

trip and immediately pull AUXGATE to ground if the

voltage between AUXIN and AUXSENSE exceeds 50mV for

more than 8

µs.TheexternalN-channeltransistoristurned

off and FAULT is pulled low. The circuit breaker is reset

when AUXON is cycled low then high, or the AUXIN supply

is cycled. If the circuit breaker feature is not required, the

AUXSENSE pin can be shorted to AUXIN.

The trip current of the circuit breaker is set by:

As a design aid, the trip current for commonly used values

for R3 is given in Table 3.

R3 (

0.05

1A

0.06

833mA

0.07

714mA

0.08

625mA

0.09

556mA

0.1

500mA

If more than 8

µs of response time is needed to reject

20

the AUXSENSE circuit. This will give a delay of 15

µs.

Supply Bypass Capacitors

In motherboard applications, large bypass capacitors are

recommended at each of the system power supplies to

minimize supply glitches as a result of board insertion. A

supply bypass capacitor of

recommended.

APPLICATIO S I FOR ATIO

Figure 8. Making PCB Connections to

the Sense Resistor for the 5V Rail

SENSE RESISTOR

CURRENT FLOW

FROM SUPPLY

CURRENT FLOW

TO LOAD

TRACK WIDTH W:

0.03" PER AMPERE

ON 1 OZ COPPER FOIL

W

4241 F08

PCB Layout Considerations for the Sense Resistor

For proper circuit breaker operation, 4-wire Kelvin-sense

connections between the sense resistor and the LTC4241’s

and AUXSENSE pins are strongly recommended. The

drawing in Figure 8 illustrates the correct way of making

connections between the LTC4241 and the sense resistor.

PCB layout should be balanced and symmetrical to mini-

mize wiring errors. In addition, the PCB layout for the

sense resistors and the power MOSFETs should include

good thermal management techniques for optimal sense

resistor power dissipation.

Power MOSFET and Sense Resistor Selection

Table 4 lists some available N-channel power MOSFETs .

Table 5 lists some current sense resistors that can be used

with the LTC4241’s circuit breakers. Table 6 lists the

supplier web site addresses for discrete components

mentioned throughout this datasheet.