LTC4380 datasheet(18/24 Pages) AD

18 / 24 page

LTC4381

Rev. 0

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APPLICATIONS INFORMATION

when VIN is minimum and load current is at its maximum

when the input transient occur (Equation 18).

P t = ILOAD • VDS • t

P t = (1A) • (100V – 10V) • 2ms

P t = 4.02W s

(18)

Next calculate the sense resistor (RSNS) value with a

current limit of greater than 1A with 10% tolerance

(Equation 19).

RSNS =

45mV

1.1• 1A

= 40.9mΩ

(19)

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

Next we select CTMR to shut off the MOSFET if the 100V

transient is longer than 2ms at maximum load of 1A

(Equation 20).

ITMR = 0.0917

⎡

⎣

⎢

⎤

⎦

⎥ • ID •RSNS •

VDS

RDRN

−70 µA

[ ]

⎛

⎝

⎜

⎞

⎠

⎟

ITMR = 0.0917

⎡

⎣

⎢

⎤

⎦

⎥ • 1A •0.04Ω•

100V –10V

82.5k

−70 µA

[ ]

⎛

⎝

⎜

⎞

⎠

⎟

= 117.2µA

(20)

Next the value is calculated using Equation 21 to achieve

a fault time of greater than 2ms:

CTMR = ITMR(UP) •

tTMR

VTMR

CTMR = 0.193µF

(21)

So we choose a CTMR = of 0.22μF. Next, we need to make

sure that the chosen CTMR allow enough time to power up

the output (Equation 22).

CTMR =

ITMR(UP) • tINRUSH

VTMR

(22)

where (Equation 23).

tINRUSH =

VIN • COUT

IINRUSH

VIN • C2

IGATE(UP)

14V • 47nF

20µA

= 32.9ms

ITMR(UP) ≈ 1.5µA at power up:

VTMR = 1.5µA •

32.9ms

0.22µF

≈ 0.224V,

(23)

which is much lower than the 1.215V trip off threshold.

Next, we need to check to make 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 24).

tOC = 0.22µF •

1.215V

80.8µA

= 3.31ms

(24)

The power dissipation in the MOSFET is given by

Equation 25.

P = 14V •

62mV

40mΩ

= 21.7W

P t

= 1.248W s

(25)

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 26.

tOC = 0.22µF •

1.215V

42.5µA

= 6.29ms

(26)

The power dissipation in the MOSFET is given by

Equation 27.

P = (14V – 3V) •

50mV

40mΩ

= 13.75W

P t = 1.09W s

(27)

These conditions are within the 10W√s safe operating

area of the MOSFET.

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