LTC4367 datasheet(20/24 Pages) AD

20 / 24 page

LTC4381

Rev. 0

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APPLICATIONS INFORMATION

Next calculate the sense resistor (RSNS) value with a current

limit of greater than 1A with 20% tolerance (Equation 31).

RSNS =

50mV

ILIM

50mV

1.2A

= 41.67mΩ

(31)

We will use 40mΩ, which gives a current limit of 1.25A.

The load dump waveform can be represented as an

exponentially decaying waveform with a time constant of

0.2sec (Equation 32).

VIN = 100 V

[ ]

e−t/0.2 s[]

(32)

The LTC4381 clamps the VOUT at 56.7V, which means

that VDS and ITMR drops to zero when VIN drops to

56.7V. To find the time t1 when this happen, we use

Equation 33.

t1 = −0.2s •In

56.7V

100V

⎛

⎝⎜

⎞

⎠⎟ =

0.113s

(33)

VDS can be approximated as a triangular waveform with

a peak of 100V – 56.7V or 43.3V and a time base of

0.113sec. We take half of the peak, 21.65V to calculate

the ITMR (Equation 34).

ITMR = 0.0917

⎡

⎣

⎢

⎤

⎦

⎥ • ID •RSNS •

VDS

RDRN

−70 µA

[ ]

⎛

⎝

⎜

⎞

⎠

⎟

ITMR = 0.0917

⎡

⎣

⎢

⎤

⎦

⎥ • 1A •0.04Ω•

21.65V

100k

−70 µA

[ ]

⎛

⎝

⎜

⎞

⎠

⎟

ITMR = 44µA

CTMR =

t1 •ITMR

VTMR

CTMR =

0.113s• 44µA

1.215V

CTMR = 4.1µF

(34)

We shall use a 4.7μF capacitor for more margin. Next, we

need to make sure that the chosen CTMR allow enough

time to power up the output (Equation 35).

CTMR =

ITMR(UP) • tINRUSH

VTMR

tINRUSH =

VIN •COUT

IINRUSH

VIN •C2

IGATE(UP)

24V • 47nF

20µA

= 56.4ms

ITMR(UP) ≈1.5µA at power up:

VTMR =

1.5µA •56.4ms

4.7µF

≈18mV

(35)

Next, we need to check to makes sure that in the case of

a severe output short where VOUT = 0V, the power dis-

sipation in the MOSFET is also within the safe operating

area (Equation 36).

tOC =

4.7µF • 1.215V

98.1µA

= 58.2ms

(36)

The power dissipation in MOSFET is given by Equation 37.

P = 24V •

62mV

40mΩ

= 37.2W

P t

= 8.97W s

(37)

During an output overload or soft short, the voltage at the

OUT pin could stay at 3V or higher. The total overcurrent

fault time when VOUT = 3V is given by Equation 38.

tOC = 4.7µF •

1.215V

60.3µA

= 94.8ms

(38)

The power dissipation in MOSFET is given by Equation 39.

P = (24V – 3) •

50mV

40mΩ

= 26.25W

P t

= 8.08W s

(39)

These conditions are within the safe operating area of

the MOSFET.

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