PS1

PS2

INN

GATE INP/VSS

INN

GATE INP/VSS

RLOAD

LM5051

LM5051

LINE

LINE

VCC

VCC

PS1

PS2

RLOAD

LM5051

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SNVS702D – OCTOBER 2011 – REVISED MARCH 2013

APPLICATION INFORMATION

FUNCTIONAL DESCRIPTION

Systems that require high availability often use multiple, parallel-connected redundant power supplies to improve

reliability. Schottky OR-ing diodes are typically used to connect these redundant power supplies to a common

point at the load. The disadvantage of using OR-ing diodes is the forward voltage drop, which reduces the

available voltage, and the associated power losses as load currents increase. Using an N-channel MOSFET to

replace the OR-ing diode requires a small increase in the level of complexity, but reduces, or eliminates, the

need for diode heat sinks or large thermal copper area in circuit board layouts for high power applications.

Figure 26. Traditional OR-ing with Diodes

The LM5051 is a negative voltage (i.e. low-side) OR-ing controller that will drive an external N-channel MOSFET

to replace an OR-ing diode. The voltage across the MOSFET source and drain pins is monitored by the LM5051

at the IN and OUT pins, while the GATE pin drives the MOSFET to control its operation based on the monitored

source-drain voltage. The resulting behavior is that of an ideal rectifier with source and drain pins of the

MOSFET acting as the anode and cathode pins of a diode respectively.

Figure 27. OR-ing with MOSFETs

INP/VSS PINS

The INP input is internally connected to the both device pin 5 and 7. Typical applications will use device pin 7

only, with a single common connection to the source connection of the N-Channel MOSFET array.

If pins 5 and 7 are both used, it is recommended that the two pins be externally connected together at the

package, with a single common connection routed to the source connection of the N-Channel MOSFET array.

Current should not be allowed flow through the internal connection between pin 5 and pin 7.

INN and GATE PINS

When power is initially applied, the load current will flow from source to drain through the body diode of the

MOSFET. The resulting voltage across the body diode will be detected across the LM5051 INN and INP/VSS

pins which then begins charging the MOSFET gate through a 0.66 mA (typical) current source.

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