PS1

PS2

IN

GATE

OUT

IN

GATE

OUT

LM5050-1

LM5050-1

GND

GND

VS

VS

PS1

PS2

LM5050-1, LM5050-1-Q1

SNVS629D – MAY 2011 – REVISED JUNE 2013

www.ti.com

APPLICATIONS 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 24. OR-ing with Diodes

The LM5050-1 is a positive voltage (i.e. high-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 LM5050-1 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 25. OR-ing with MOSFETs

IN, GATE AND OUT 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 at the LM5050-1 IN and OUT pins which

then begins charging the MOSFET gate through a 32 µA (typical) charge pump current source . In normal

operation, the gate of the MOSFET is charged until it reaches the clamping voltage of the 12V GATE to IN pin

zener diode internal to the LM5050-1.

The LM5050-1 is designed to regulate the MOSFET gate- to -source voltage if the voltage across the MOSFET

10

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