LTC4355 LINER [Linear Technology], LTC4355 Datasheet - Page 7

• Manufacturer Part Number: LTC4355
• Description: Positive High Voltage Ideal Diode-OR with Input Supply and Fuse Monitors
• Manufacturer: LINER [Linear Technology]

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 efficiency loss. This drop reduces
the available supply voltage and dissipates significant
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 transis-
tors 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 flows 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 amplifier 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-
lation. The current flowing through each pass trans-
DS(ON)
• I
LOAD
.
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 flows 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
configurable fault threshold, DV
pulls low. Using this pin to shunt current away from an
LED or optocoupler 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
optocoupler.
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
, the VDSFLT pin
LTC4355
< 1.23V, the
4355fa