LT4351CMS datasheet(8/20 Pages) LINER

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LT4351

sn4351 4351fs

OPERATIO

Increasingly, system designers have to deal with multiple

supply sources. The multiplicity may provide parallel, re-

dundant supplies for increased reliability or provide a means

of connecting disparate supplies. In all cases the desire is

for behavior like a diode but with no loss or voltage drop.

ORing diodes have been the conventional means of

connecting these supplies. The disadvantage of this ap-

proach is that diodes introduce efficiency loss because of

their forward voltage drop. This variable voltage drop also

degenerates supply tolerance. Additionally, diodes pro-

vide no information concerning the status of the sourcing

supply. Separate control must also be added to ensure

that a supply that is out of range is not allowed to affect

the common supply.

The LT4351 eliminates these problems by using N-channel

MOSFETs as the pass elements. The MOSFET is turned on

when power is being passed, allowing for a low voltage

drop from the supply to the load. When the input source

voltage drops below the output common supply voltage it

turns off the MOSFET, thereby matching the function and

performance of an ideal diode.

The LT4351 drives either a single MOSFET or dual back-

to-back MOSFETs. Dual MOSFETs are chosen to eliminate

current flow from the input supply to the output supply

when the VIN voltage is greater than OUT.

A driver amplifier monitors the input (VIN) and output

(OUT) and controls the MOSFETs. If VIN exceeds OUT by

15mV, GATE goes high and turns on the MOSFET(s)

allowing for power passage.

Undervoltage and overvoltage comparators CUV, COV and

COVF also control power passage. A resistive divider in

conjunction with the UV and OV pins sets appropriate

thresholds such that the MOSFET(s) is off when the UV pin

is below 300mV or OV pin is above 300mV.

To help deal with the transients on the supply lines, the UV

input has current hysteresis. When the UV voltage drops

below the 300mV threshold, a 10

µA current is pulled from

the pin. Thus the user can set the hysteresis level through

appropriate values in the divider.

Overvoltage shutdown occurs in two stages. The first

occurs when the OV pin exceeds the 300mV reference.

When OV just exceeds the reference, an internal capacitor

starts charging, delaying the signal to turn off the

MOSFET(s).

The second occurs when the OV pin exceeds 330mV. The

OVF comparator will immediately trip pulling GATE to

GND. This affords a delay inversely proportional to the

amount of overdrive. This also provides for glitch immu-

nity without compromising response time in the event of

a serious overvoltage condition.

The FAULT output indicates the status of the COV, COVF and

CUV comparators. It pulls low during a fault condition. It

also pulls low when GATE is at compliance and VIN > OUT

by more than 0.21V indicating a probable nonfunctioning

MOSFET. Compliance occurs when GATE is at the lesser

of OUT + 7.4V or VDD – 2.3V. FAULT derives its drive from

the greater of VIN or OUT. It is active if VIN or OUT is greater

than 0.9V. If VIN or OUT is below this level, the output state

is not guaranteed.

The gate drive consists of a high current, wide bandwidth

amplifier (Driver). When the amplifier is enabled, it at-

tempts to regulate the GATE voltage such that the voltage

across the MOSFET(s) is approximately 15mV. If the

MOSFET(s) on resistance is so high as to prevent regula-

tion, then GATE goes to compliance and the MOSFET(s)

fully turns on. The inputs to the amplifier are VIN and OUT.

The GATE pin sources current from VDD and sinks current

to GND. The maximum GATE to VIN voltage is the lesser of

VDD – 2.3V or 7.4V above VOUT or VIN (internal clamp

voltage).

The STATUS comparator, ST, pulls low when GATE ex-

ceeds VIN by 0.7V. This occurs when VIN > OUT + 15mV.

The STATUS pin pulls low as an indication that power is

passing through the MOSFET(s).

If VIN is greater than OUT by 0.21V and GATE > VIN + 7.4V

or at compliance (GATE = VDD – 2.3V), STATUS will go

high as an indication of a likely open MOSFET. FAULT will

pull low in this state indicating the probable fault.

The gate drive amplifier and STATUS function derive

power from VDD. The circuit requires VDD > 2.5V. If VDD is

present, the gate drive amplifier and STATUS are active

independent of the state of VIN. If in a fault, GATE pulls

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