LTC4355CS datasheet(7/20 Pages) LINER

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LTC4355

4355fe

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

transistors 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 RDS(ON) • ILOAD.

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-

istor depends on the RDS(ON) of each MOSFET and the

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,

ΔVSD(FLT), the VDSFLT pin

pulls low. Using this pin to shunt current away from an

LED or opto-coupler 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 VMON1 < 1.23V or VMON2 < 1.23V, the

corresponding PWRFLT pin pulls low to indicate that the

input supply is low, turning off an optional LED or opto-

coupler.

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.

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