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LT4363

4363fa

(SOA), an intrinsic property of the MOSFET. SOA quanti-

fies 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 is expressed in units of

watt-squared-seconds(P2t).Thisfigureisessentiallycon-

stant 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

P2t is not the same for all combinations of ID and VDS.

In particular P2t tends to degrade as VDS approaches the

maximum rating, rendering some devices useless for

absorbing energy above a certain voltage.

When a fast input voltage step occurs, the current through

thepasstransistortosupplytheloadandchargeuptheout-

put capacitor can be high enough to trigger an overcurrent

event. The gate pulls low to 1V above the OUT pin, turning

off the MOSFET momentarily. The internal charge pump

will then start to pull the GATE pin high and turn on the

MOSFET to support the load current and charge up the

OUT pin. The fault timer may not start yet because the

current level is below the overcurrent limit threshold and

the output voltage has not reached the servo voltage. This

extra stress needs to be included in calculating the overall

stress level of the MOSFET.

Calculating Transient Stress

To select a MOSFET suitable for any given application, the

SOA stress must be calculated for each input transient

whichshallnotinterruptoperation.Itisthenasimplematter

to choose a device which has adequate SOA to survive the

maximumcalculatedstress.P2tforaprototypicaltransient

waveform is calculated as follows (Figure 4):

Let

a = VREG – VIN

b = VPK – VIN

(VIN = Nominal Input Voltage)

Then

P2t = ILOAD2 •

b– a

(

τ 2a2 ln

+3a2 +b2 − 4ab





















Typically VREG ≈ VIN and τ » tr simplifying the above to

P2t

ILOAD2 VPK – VREG

(

)2τ [W2s]

For the transient conditions of VPK = 80V, VIN = 12V,

VREG = 16V, tr = 10µs and τ = 1ms, and a load current

of 3A, P2t is 18.4W2s – easily handled by a MOSFET in

a DPAK package. The P2t of other transient waveshapes

is evaluated by integrating the square of MOSFET power

over time. LTSpice can be used to simulate timer behavior

for more complex transients and cases where overvoltage

and overcurrent faults coexist.

Calculating Short-Circuit Stress

SOA stress must also be calculated for short-circuit condi-

tions. Short-circuit P2t is given by:

P2t

= ΔVDS •

ΔVSNS

RSNS









• tTMR [W

2s]

Where ∆VDS is the voltage across the MOSFET, and ∆VSNS

is the SNS pin threshold, and tTMR is the overcurrent timer

interval.

For VIN = 15V, ∆VDS = 13V (VOUT = 2V), ∆VSNS = 50mV,

RSNS = 12mΩ and CTMR = 100nF, P2t is 6.3W2s – less

than the transient SOA calculated in the previous example.

Nevertheless, to account for circuit tolerances this figure

should be doubled to 12.6W2s.

APPLICATIONS INFORMATION

Figure 4. Safe Operating Area Required to Survive Prototypical

Transient Waveform

VPK

VIN

4363 F04

VREG

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