LTC4355IS#TRPBF Linear Technology, LTC4355IS#TRPBF Datasheet - Page 7

• Manufacturer Part Number: LTC4355IS#TRPBF
• Description: IC IDEAL DIODE-OR 16-SOIC
• Manufacturer: Linear Technology

Specifications of LTC4355IS#TRPBF

• Applications: -48V Dist Power Systems, AdvancedTCA ® Systems
• Fet Type: N-Channel
• Number Of Outputs: 2
• Internal Switch(s): No
• Delay Time - Off: 300ns
• Voltage - Supply: 9 V ~ 80 V
• Current - Supply: 2mA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Delay Time - On: -

OPERATION
High availability systems often employ parallel-connected
power supplies or battery feeds to achieve redundancy
and enhance system reliability. ORing diodes have been
a popular means of connecting these supplies at the
point of load. The disadvantage of this approach is the
forward voltage drop and resulting effi ciency loss. This
drop reduces the available supply voltage and dissipates
signifi cant power. Using N-channel MOSFETs to replace
Schottky diodes reduces the power dissipation and
eliminates the need for costly heat sinks or large thermal
layouts in high power applications.
The LTC4355 is a positive voltage diode-OR controller
that drives two external N-channel MOSFETs as pass
transistors to replace ORing diodes. The IN and OUT
pins form the anodes and cathodes of the ideal diodes.
The source pins of the external MOSFETs are connected
to the IN pins. The drains of the MOSFETs are connected
together at the OUT pin, which is the positive supply of
the device. The gates of the external MOSFETs are driven
by the LTC4355 to regulate the voltage drop across the
pass transistors.
At power-up, the initial load current fl ows through the
body diode of the MOSFET with the higher INx voltage.
The associated GATEx pin immediately ramps up and
turns on the MOSFET. The amplifi er tries to regulate the
voltage drop across the source and drain connections to
25mV. If the load current causes more than 25mV of drop,
the MOSFET gate is driven fully on and the voltage drop
is equal to R
When the power supply voltages are nearly equal, this
regulation technique ensures that the load current is
smoothly shared between the MOSFETs without oscil-
DS(ON)
• I
LOAD
.
lation. The current fl owing through each pass trans-
istor depends on the R
output impedances of the supplies.
In the event of a supply failure, such as if the supply that
is conducting most or all of the current is shorted to GND,
reverse current fl ows temporarily through the MOSFET that
is on. This current is sourced from any load capacitance
and from the second supply through the body diode of
the other MOSFET. The LTC4355 quickly responds to this
condition, turning off the MOSFET in about 500ns. This
fast turn-off prevents the reverse current from ramping
up to a damaging level.
In the case where the forward voltage drop exceeds the
confi gurable fault threshold, ΔV
pulls low. Using this pin to shunt current away from an
LED or opto-coupler provides an indication that a pass
transistor has either failed or has excessive forward current.
Additionally, in this condition the PWRFLT1 or PWRFLT2
pin pulls low to identify the faulting channel.
The PWRFLT pins also indicate if an input supply is within
regulation. When V
corresponding PWRFLT pin pulls low to indicate that the
input supply is low, turning off an optional LED or opto-
coupler.
The FUSEFLT pins indicate the status of input fuses. If
the voltage at one of the IN pins is less than 3.5V, the
corresponding FUSEFLT pin pulls low. The IN pins sink
a minimum of 0.5mA to guarantee that the IN pin will
pull low when the input fuse is blown open. Note that the
FUSEFLT pin will activate if the input supply is less than
3.5V even if the fuse is intact.
MON1
DS(ON)
< 1.23V or V
of each MOSFET and the
SD(FLT)
MON2
LTC4355
, the VDSFLT pin
< 1.23V, the
4355fe
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