LTC4380 datasheet(14/24 Pages) AD

14 / 24 page

LTC4381

Rev. 0

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APPLICATIONS INFORMATION

by using short, wide traces. An RC filter at the VCC pin is

an effective measure against voltage spikes.

Another way to limit transients to less than 80V at the VCC

pin is to use a small Zener diode and a resistor, D1 and R1

in Figure 1. The Zener diode limits the voltage at the pin

while the resistor limits the current through the diode to a

safe level during the surge. However, D1 can be omitted if

the filtered voltage at the VCC pin, due to R1 and C1, stays

below 80V. The inclusion of R1 in series with the VCC pin

modestly increases the minimum required voltage at VIN

due to the extra voltage drop across it from the small VCC

current of the LTC4381 and the leakage current of D1.

A total bulk capacitance of at least 22µF low ESR electro-

lytic or ceramic is required close to the OUT pin.

Transient Stress in the MOSFET

During an overvoltage event, the LTC4381 clamps the

gate of the pass MOSFET to limit the output voltage at an

acceptable level. The load circuitry may continue operat-

ing throughout this interval, but only at the expense of

dissipation in the MOSFET pass device. MOSFET dissipa-

tion or stress is a function of the input voltage waveform,

output voltage and load current.

Most transient event specifications use the prototypi-

cal waveshape shown in Figure 2, comprising a linear

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

exponentially decaying back to VIN with a time constant

of τ. A common automotive transient specification has

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

condition known as load dump commonly has constants

of tr = 5ms, VPK = 60V and τ = 200ms.

MOSFET stress is the result of power dissipated within

the device. For long duration surges of 100ms or more,

stress is increasingly dominated by heat transfer out of

the package; this is a matter of device packaging and

mounting and heat sink thermal mass. This is best ana-

lyzed by simulation using the MOSFET thermal model.

For short duration transients of less than 100ms, MOSFET

survival is a matter of safe operating area (SOA), an

intrinsic property of the MOSFET. SOA quantifies the

time required at any given condition of VDS and ID to

raise the junction temperature of the MOSFET to its rated

maximum. MOSFET SOA can be expressed in units of

watt-root-seconds (P√t), which is essentially constant

for intervals of less than 100ms for any given device

type and rises to infinity under DC operating conditions.

Destruction mechanisms other than bulk die temperature

distort the lines of an accurately drawn SOA graph so that

P√t is not the same for all combinations of ID and VDS.

In particular P√t tends to degrade as VDS approaches

the maximum rating, rendering some devices useless for

absorbing energy above a certain voltage. The LTC4381

internal MOSFET has a guaranteed SOA of 20ms at 70V

and 1A, which gives a P√t of 10W√s. To survive a longer

overvoltage transient, reduce the load current according

to this P√t spec.

Figure 2. Prototypical Transient Waveform

VPK

VIN

4381 F02

Figure 3. Safe Operating Area Required to Survive

Prototypical Transient Waveform

VPK

VIN

4381 F03

VREG

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