LTC4358 datasheet(6/14 Pages) AD

6 / 14 page

LTC4450

Rev. 0

For more information www.analog.com

OPERATION

The LTC4450 is an 18V Schottky diode replacement con-

sisting of an integrated N-channel power MOSFET and

controller. The ideal diode regulates the forward voltage to

15mV to minimize power loss and ensure smooth current

transfer without oscillation in diode-OR applications. The

power MOSFET can deliver up to 12A output current from

its input supply with a forward conduction resistance of

7mΩ when fully enhanced.

The LTC4450 features a high performance precision

OTA which senses the voltage on the IN and OUT pins

and drives the power MOSFET gate in regulation. The

OTA senses when the forward voltage drop across the

power MOSFET is sufficiently large and drives its gate

to achieve full enhancement within 1µs. When the power

MOSFET is fully enhanced, the forward voltage drop is

equal to RDS(ON) • IOUT. The OTA also senses when reverse

conditions are present and drives the gate of the power

MOSFET to IN within 1µs to disable the ideal diode allow-

ing the LTC4450 to have a fast reverse recovery time.

The STATUS open-drain output pulls low when the gate

of the N-Channel MOSFET is pulled low indicating that

the LTC4450 operates in reverse bias or in shutdown.

Otherwise, STATUS pulls high indicating that the LTC4450

is operating in forward bias. Connect STATUS to an

external supply through a pull-up resistor. Leave open

when unused.

When the SHDN# pin is pulled below 0.4V (VSHDN#(TH)),

the gate drive for the power MOSFET is disabled and the

LTC4450 enters a low current state. STATUS is pulled low

and the power MOSFET body diode conducts the load

current under forward biased condition. A SHDN# low

to high transition allows the LTC4450 to power up and

operate normally.

IN can operate down to 0V when the VCC auxiliary supply

voltage is above 2.75V. When either IN or VCC exceeds

2.75V the LTC4450 activates an internal burst-mode

charge pump to drive the gate of the N-Channel power

MOSFET. The burst-mode charge pump requires tSTART to

turn on. During that delay the power MOSFET body diode

conducts the load and inrush current if forward biased.

APPLICATIONS INFORMATION

Blocking diodes, commonly implemented by Schottky

diodes, are typically placed in series with supply inputs

for the purpose of ORing redundant power sources and

protecting against supply reversal. The LTC4450 replaces

diodes in these applications. Under forward conduction

both the voltage drop and power loss are greatly reduced

compared to a passive solution. When blocking voltage

under reverse bias, the LTC4450 has significantly less

reverse leakage than a typical Schottky diode.

The LTC4450 has a wide operating voltage range of 2.75V

to 18V. When an auxiliary supply voltage is connected to

VCC the ideal diode can operate down to 0V making it ideal

for applications that must tolerate large voltage transients.

Ideal diodes, like their non-ideal counterparts, exhibit a

behavior known as reverse recovery. The presence of par-

asitic input inductance may cause large and potentially

damaging reverse recovery current spikes during a reverse

mode commutation. Spikes and protection schemes

are discussed in detail in the Input Short-Circuit Faults

section.

It is important to note that the SHDN# pin, while disabling

the LTC4450 and reducing its current consumption, does

not disconnect the load from the input since the internal

MOSFET’s body diode is ever-present.

Paralleling Supplies (Diode-OR)

In many electrical systems, it is common to have a backup

power source in addition to the primary power source.

When the primary power source droops or is removed,

the system runs from the backup power source. The

outputs of two or more LTC4450s can be combined for

redundancy or for droop sharing as shown in Figure 1.

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