LTC4215 datasheet(14/20 Pages) LINER

14 / 20 page

LTC4234

4234fa

For more information www.linear.com/LTC4234

The inrush current is defined by the current required to

chargetheoutputcapacitorusingthefixed0.35V/msGATE

charge up rate. The inrush current is defined as:

IINRUSH =CL •

0.35V











 =1000µF•

0.35V











 =350mA

As mentioned previously the charge-up time is the output

voltage (12V) divided by the output rate of 0.35V/ms

resulting in 34ms. The peak power dissipation of 12V at

350mA (or 4.2W) must not exceed the SOA of the pass

MOSFET for 34ms (see MOSFET SOA graph in the Typical

PerformanceCharacteristics).OntheSOAgraphthe30ms

line crosses the 10V VDS vertical line at 8A. This verifies

that the 80W for 30ms is safe at room temperature. Each

single point on the 8ms and 30ms lines represent a power

(voltage times current) and time that follow a constant P2t

relationship of 200W2s. This constant P2t number is valid

for power pulses less than 50ms. Beyond 50ms the P2t

number will depend on the thermal characteristics of the

board. If the MOSFET junction temperature is elevated,

then the P2t constant must be derated. At TJ = 60°C the

new constant becomes:

P2t TJ = 60°C

(

)=200 W2s







 •

150

°C–60°C

150

°C–25°C









104 W2s









The maximum power for 34ms can be calculated from

the derated constant:

PMAX =

P2t TJ = 60°C

(

)

104 W2s









34ms

= 55W

Therefore the power dissipation at charge-up is within

the MOSFET SOA.

Next the power dissipated in the MOSFET during overcur-

rent must be limited. The active current limit uses a timer

to prevent excessive energy dissipation in the MOSFET.

Theworst-casepowerdissipationoccurswhenthevoltage

versus current profile of the foldback current limit is at the

maximum. This occurs when the current is 25A and the

voltage is one-half of the VIN or 6V. See the Current Limit

Threshold Foldback in the Typical Performance Charac-

teristics section to view this profile. In order to survive

150W, the MOSFET SOA dictates a maximum current limit

timeout. If the MOSFET operating temperature is elevated

prior to current limit the SOA constant must be derated

according to the formula:

P2t TJ

( )=P2t 25°C

(

)• 150°C–TJ

150

°C–25°C









TJ is calculated from the ambient temperature, package

thermal impedance (

θJA) and the I2R heating:

TJ = (θJA • I2 • RON) + TA = 15°C/W • (20A)2 • 7.2mΩ

+ 60°C = 103˚C

Use the SOA derating formula:

P2t TJ =103°C

(

)=200 W2S







•

150

°C–103°C

150

°C–25°C









= 28 W2S









So the SOA constant is derated to 28 W2s. The maximum

currentlimittimeoutiscalculatedfromtherevisedconstant

and the 150 W dissipated in current limit:

tMAX =

P2t TJ =103°C

(

)

28 W2S









150W

(

= 1.2ms

APPLICATIONS INFORMATION

ADC

226k

1µF

20k

*TVS Z1: DIODES INC. SMAJ17A

12V

4234 F05

68nF

1000µF

VOUT

12V

20A

VDD

Z1*

OUT

GND

IMON

RMON

20k

ISET

GATE

LTC4234

INTVCC

TIMER

SENSE–

SENSE

FLT

140k

20k

10k

20k

150k

UV = 9.88V

OV = 15.2V

PG = 10.5V

Figure 5. 20A, 12V Card Resident Application

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