LTC4354

6

4354f

TI I G DIAGRA

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

transistors to replace ORing diodes. The MOSFETs are

connected together at the source pins. The common

negative supply of the device. It is also connected to the

positive inputs of the amplifiers that control the gates to

regulate the voltage 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 gate is

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 de-

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

second 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

optocoupler 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

supply transients and supplies AC current to the device.

OPERATIO

4354 TD01

400mV

2V

–100mV