LTC4358
6
4358fa
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 an N-channel MOSFET to replace a Schottky
diode reduces the power dissipation and eliminates the
need for costly heat sinks or large thermal layouts in high
power applications.
The LTC4358 is a single positive voltage ideal diode con-
troller that drives an internal N-channel MOSFET as a pass
transistor to replace a Schottky diode. The IN and DRAIN
pins form the anode and cathode of the ideal diode. The
input supply is connected to the IN pins, while the DRAIN
pin serves as the output. The OUT pin is connected directly
to DRAIN and V
DD
. V
DD
is the supply for the LTC4358 and
is derived from the output either directly or through an
RC hold-up circuit.
At power-up, the load current initially fl ows through the
body diode of the internal MOSFET. The internal MOSFET
turns on and the amplifi er tries to regulate the voltage
drop across the IN and OUT connections to 25mV. If
the load current causes more than 25mV of drop, the
MOSFET is driven fully on and the voltage drop is equal to
R
DS(ON)
• I
LOAD
.
If the load current is reduced causing the forward drop to
fall below 25mV, the internal MOSFET is driven lower by
a weak pull-down in an attempt to maintain the drop at
25mV. If the load current reverses the MOSFET is turned
off with a strong pull-down.
In the event of a power supply failure, such as if the sup-
ply that is conducting most or all of the current is shorted
to ground, reverse current temporarily fl ows through the
LTC4358 ideal diode that is on. This current is sourced
from any load capacitance and from the other supplies.
The ideal diode is turned off within 500ns, preventing
reverse current from slewing up to a damaging level and
minimizing any disturbance on the output.
