LTC4241
14
sn4241 4241f
Table 4. N-Channel Power MOSFET Selection Guide
CURRENT PART NUMBER PACKAGE V
DS
MAX V
GS
MAX R
DS(on)
MANUFACTURER
RATING
8.0A Si4412ADY SO-8 30V ±20V 0.024Ω Vishay-Siliconix
3.5A Si2306DS SOT-23 30V ±20V 0.057Ω Vishay-Siliconix
10A Si4410DY SO-8 30V ±20V 0.013Ω Vishay-Siliconix
13A IRF7413 SO-8 30V ±20V 0.011Ω International Rectifier
2.7A FDN 359AN SOT-23 30V ±20V 0.046Ω 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 8µs. 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 8µs. The external N-channel transistor is turned
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:
I
TRIP
= 50mV/R3
As a design aid, the trip current for commonly used values
for R3 is given in Table 3.
Table 3. I
TRIP
vs R3
R3 (Ω)I
TRIP
0.05 1A
0.06 833mA
0.07 714mA
0.08 625mA
0.09 556mA
0.1 500mA
If more than 8µs of response time is needed to reject
supply current ripple noise, an external resistor, R
F
, of
20Ω and capacitor, C
F
, of 1µF (Figure 6) can be added to
the AUXSENSE circuit. This will give a delay of 15µs.
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 ≥100µF at 12V
IN
connection is
recommended.
APPLICATIO S I FOR ATIO
WUUU
Figure 8. Making PCB Connections to
the Sense Resistor for the 5V Rail
SENSE RESISTOR
CURRENT FLOW
FROM SUPPLY
CURRENT FLOW
TO LOAD
5V
IN
5V
SENSE
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
5V
IN
and 5V
SENSE
pins, 3V
IN
and 3V
SENSE
pins and AUXIN
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.
