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LT3740

3740fc

APPLICATIONS INFORMATION

Selecting a standard value of 2.0μH results in a ripple

current of:

ΔI

L =

2.5V

(300kHz) •(2

μH)

•1–

2.5V

15V

⎛

⎝⎜

⎞

⎠⎟

= 3.47A

Set Range = VIN. At minimum input voltage VIN = 7V,

the maximum current sensing voltage is 145mV. Use an

external sensing resistor of 12mΩ. The inductor current

valley will be clamped to 12A. The ripple current at VIN= 7V

is 2.68A, there is about 33% of margin for the 10A load

current to cover the maximum current sensing voltage

variation.

For the case of using MOSFET on-resistance for current

sensing, choosing a Si4840 (RDS(ON) = 0.008Ω (NOM)

0.0095Ω (MAX) for VGS = 7V, θJA = 40°C/W) yields a

nominal sense voltage of:

VSN(NOM) = (10A)(1.3)(0.0095Ω) =123mV

To check if the current limit is acceptable, assume a junc-

tion temperature of about 55°C above a 70°C ambient

with

ρ125°C = 1.5:

LIMIT =

145mV

1.5 • 0.0095

2.68A

= 11.5A

Double check the assumed TJ in the MOSFET at VIN = 7V

with maximum load current:

PBOT = DBOT • IL2 • RDS(ON), BOT

BOT = 1–

2.5V

⎛

⎝⎜

⎞

⎠⎟

•10A

()2 •1.5•0.0095=0.92W

Double check the assumed TJ in the MOSFET:

TJ = 70°C + (0.92W)(40°C/W) = 107°C

The power dissipation in the bottom MOSFET increases

with input voltage. For VIN = 20V,

BOT = 1–

2.5V

20V

⎛

⎝⎜

⎞

⎠⎟

•10A

()2 •1.5•0.0095Ω=1.25W

Double check the assumed TJ in the MOSFET:

TJ = 70°C + (1.25W)(40°C/W) = 120°C

Choose a Si4840 (CRSS = 200pF) for the top MOSFET and

check its power dissipation at maximum load current with

ρ100°C = 1.3:

TOP =

2.5V

20V

•10A

()2 •1.3•0.0095Ω+17•(20V)2 •

10A • 200pF • 300kHz

= 0.15W + 0.41W = 0.56W

TJ = 70°C + (0.56W)(40°C/W) = 92°C

This analysis shows that careful attention to heat sinking

will be necessary in this circuit.

Check the reverse current comparator margin. The maxi-

mum ripple current happens at maximum input voltage:

ΔI

L(MAX) =

2.5V

300kHz • 2

μH

•1–

2.5V

20V

⎛

⎝⎜

⎞

⎠⎟

= 3.65A

At no load, the maximum reverse current voltage is:

S •

ΔI

L(MAX)

= 12mΩ •

3.65A

= 22mV

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