LTC4357 LINER [Linear Technology], LTC4357 Datasheet - Page 6

• Manufacturer Part Number: LTC4357
• Description: Positive High Voltage Ideal Diode Controller
• Manufacturer: LINER [Linear Technology]

OPERATION
LTC4357
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 LTC4357 controls an external N-channel MOSFET to
form an ideal diode. The voltage across the source and
drain is monitored by the IN and OUT pins, and the GATE
pin drives the MOSFET to control its operation. In effect
the MOSFET source and drain serve as the anode and
cathode of an ideal diode.
At power-up, the load current initially fl ows through the
body diode of the MOSFET. The resulting high forward
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voltage is detected at the IN and OUT pins, and the
LTC4357 drives the GATE pin to servo the forward drop
to 25mV. If the load current causes more than 25mV of
voltage drop when the MOSFET gate is driven fully on,
the forward voltage is equal to R
If the load current is reduced causing the forward drop
to fall below 25mV, the MOSFET gate is driven lower by
a weak pull-down in an attempt to maintain the drop at
25mV. If the load current reverses and the voltage across
IN to OUT is more negative than –25mV the LTC4357
responds by pulling the MOSFET gate low with a strong
pull-down.
In the event of a power supply failure, such as if the output
of a fully loaded supply is suddenly shorted to ground,
reverse current temporarily fl ows through the MOSFET that
is on. This current is sourced from any load capacitance
and from the other supplies. The LTC4357 quickly responds
to this condition turning off the MOSFET in about 500ns,
thus minimizing the disturbance to the output bus.
DS(ON)
• I
LOAD
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