LTC2913 datasheet(12/24 Pages) AD

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LT4356-3

Rev D

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The SOA of the MOSFET must encompass all fault condi-

tions. In normal operation the pass transistor is fully on,

dissipating very little power. But during either overvoltage

or overcurrent faults, the GATE pin is servoed to regu-

late either the output voltage or the current through the

MOSFET. Large current and high voltage drop across the

MOSFET can coexist in these cases. The SOA curves of

the MOSFET must be considered carefully along with the

selection of the fault timer capacitor.

Transient Stress in the MOSFET

During an overvoltage event, the LT4356-3 drives a series

passMOSFETtoregulatetheoutputvoltageatanacceptable

level.Theloadcircuitrymaycontinueoperatingthroughout

this interval, but only at the expense of dissipation in the

MOSFET pass device. MOSFET dissipation or stress is a

function of the input voltage waveform, regulation voltage

and load current. The MOSFET must be sized to survive

this stress.

Most transient event specifications use the model shown

in Figure 3. The idealized waveform comprises a linear

ramp of rise time tr, reaching a peak voltage of VPK and

exponentially decaying back to VIN with a time constant

of t. A common automotive transient specification has

constants of tr = 10µs, VPK = 80V and t = 1ms. A surge

conditionknownas“loaddump”hasconstantsoftr = 5ms,

VPK = 60V and t = 200ms.

This fixed early warning period allows time for the system

to perform necessary backup or house-keeping functions

before power is cut off. When VTMR crosses the 1.35V

threshold, the GATE pin pulls low immediately and turns

off the MOSFET. Note that during an overcurrent event the

timer current is not reduced to 5µA when VTMR reaches

1.25V,sinceitwouldlengthentheoverallfaulttimerperiod

and cause additional MOSFET stress. After the GATE pin

pulls low due to a fault time out, the LT4356-3 latches off.

Allow sufficient time for the TMR pin to discharge to 0.5V

(typical discharge current is 2.2µA) and for the MOSFET

to cool before attempting to reset the part. To reset, pull

the SHDN pin low for at least 100µs, then pull high with

a slew rate of at least 10V/ms.

MOSFET Selection

TheLT4356-3drivesanN-channelMOSFETtoconductthe

load current. The important features of the MOSFET are

on-resistanceRDS(ON),themaximumdrain-sourcevoltage

V(BR)DSS, the threshold voltage, and the SOA.

The maximum allowable drain-source voltage must be

higher than the supply voltage. If the output is shorted

to ground or during an overvoltage event, the full supply

voltage will appear across the MOSFET.

The gate drive for the MOSFET is guaranteed to be more

than 10V and less than 16V for those applications with VCC

higher than 8V. This allows the use of standard threshold

voltage N-channel MOSFETs. For systems with VCC less

than 8V, a logic level MOSFET is required since the gate

drive can be as low as 4.5V.

APPLICATIONS INFORMATION

Figure 3. Prototypical Transient Waveform

VPK

VIN

43563 F03

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