LTC4449 datasheet(32/38 Pages) LINER

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LTC3774

3774fc

For more information www.linear.com/LTC3774

The maximum DCR of the inductor is 0.34mΩ. The

VSENSE(MAX) is calculated as:

VSENSE(MAX) = IPEAK • DCRMAX = 12mV

The current limit is chosen to be 15mV. If temperature

variation is considered, please refer to Inductor DCR

SensingTemperatureCompensationwithNTCThermistor.

The power dissipation on the topside MOSFET can be

easily estimated. Choosing an Infineon BSC050NE2LS

MOSFET results in: RDS(ON) = 7.1mΩ (max), VMILLER =

2.8V, CMILLER ≅ 108pF. At maximum input voltage with

TJ (estimated) = 75°C:

PMAIN =

1.5V

20V

30A

(

)

2 1+(0.005)(75°C – 25°C)

[

] •

0.0071Ω

(

)+ 20V

(

)

2 30A

⎛

⎝

⎜

⎞

⎠

⎟ 2Ω

( ) 108pF

(

) •

5.5V – 2.8V

2.8V

⎡

⎣⎢

⎤

⎦⎥

400kHz

(

)

= 599mW + 377mW

= 976mW / phase

For a 0.32mΩ DCR, a short-circuit to ground will result

in a folded back current of:

ISC =

1/ 3

(

)15mV

0.00032Ω

– 1

90ns(20V)

0.33µH

⎛

⎝

⎜

⎞

⎠

⎟ = 12.9A / phase

An Infineon BSC010NE2LS, RDS(ON) = 1.1mΩ, is chosen

for the bottom FET. The resulting power loss is:

PSYNC =

20V – 1.5V

20V

30A

(

)

2 •

1+ 0.005

(

) • 75°C – 25°C

(

)

⎡⎣

⎤⎦• 0.0011Ω

PSYNC = 1.14W/phase

CIN is chosen for an equivalent RMS current rating of at

least 13.7A. COUT is chosen with an equivalent ESR of

4.5mΩ for low output ripple. The output ripple in continu-

ous mode will be highest at the maximum input voltage.

The output voltage ripple due to ESR is approximately:

VORIPPLE = RESR (∆IL) = 0.0045Ω • 10A = 45mVP-P

Further reductions in output voltage ripple can be made

by placing a 100µF ceramic capacitor across COUT.

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