LTC4354
6
4354fc
TiMing DiagraM
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
significant forward-voltage drop and resulting efficiency
loss. This drop reduces the available supply voltage and
dissipates significant power. A desirable circuit would
behave like diodes but without the voltage drop and the
resulting power dissipation.
The LTC4354 is a negative voltage diode-OR controller that
drives two external N-channel MOSFETs as pass transis-
tors to replace ORing diodes. The MOSFETs are connected
together at the source pins. The common source node is
connected to the V
SS
pin which is the negative supply of
the device. It is also connected to the positive inputs of
the amplifiers that control the gates to regulate the volt-
age drop across the pass transistors. 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.
At power-up, the initial load current flows through the
body diode of the MOSFET
and returns to the supply with
the
lower terminal voltage. The associated gate pin will
immediately start ramping up and turn on the MOSFET.
The amplifier tries to regulate the voltage drop between
the source and drain connections to 30mV. If the load
current causes more than 30mV of drop, the gate rises
to further enhance the MOSFET. Eventually the MOSFET
4354 TD01
V
SS
– V
DX
V
GATE
t
OFF
100mV
2V
–400mV
operaTion
gate is driven fully on and the voltage drop is equal to the
R
DS(ON)
• I
LOAD
.
When the power supply voltages are nearly equal, this
regulation technique ensures that the load current is
smoothly shared between them without oscillation. The
current level flowing through each pass transistor depends
on the R
DS(ON)
of the MOSFET and the output impedance
of the supplies.
In the case of supply failure, such as if the supply that
is conducting most or all of the current is shorted to the
return side, a large reverse current starts flowing through
the MOSFET that is on, from any load capacitance and
through the body diode of the other MOSFET, to the sec-
ond supply. The LTC4354 detects this failure condition as
soon as it appears and turns off the MOSFET in less than
1µs. This fast turn-off prevents the reverse current from
ramping up to a damaging level.
In the case where the pass transistor is fully on but the
voltage drop across it exceeds the fault threshold, the
FAULT pin goes high impedance. This allows an LED or
opto-coupler to turn on indicating that one or both of the
pass transistors have failed.
The LTC4354 is powered from system ground through a
current limiting resistor. An internal shunt regulator that
can sink up to 20mA clamps the V
CC
pin to 11V above V
SS
.
A 1µF bypass capacitor across V
CC
and V
SS
pins filters
supply transients and supplies AC current to the device.
