LTC3707IGN datasheet(26/32 Pages) LINER

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LTC3707

3707fb

APPLICATIONS INFORMATION

Design Example

As a design example for one channel, assume VIN =

12V(nominal), VIN = 22V(max), VOUT = 2V, IMAX = 5A,

and f = 300kHz.

The inductance value is chosen ﬁrst based on a 30% ripple

current assumption. The highest value of ripple current

occurs at the maximum input voltage. Tie the FREQSET

pin to the INTVCC pin for 300kHz operation. The minimum

inductance for 30% ripple current is:

ΔI

L =

OUT

(f)(L)

1–

OUT

⎛

⎝⎜

⎞

⎠⎟

A 4.7μH inductor will produce 25% ripple current and a

3.3μH will result in 36%. The peak inductor current will be

the maximum DC value plus one half the ripple current, or

5.92A, for the 3.3μH value. Increasing the ripple current

will also help ensure that the minimum on-time of 200ns

is not violated. The minimum on-time occurs at maximum

VIN:

ON(MIN) =

OUT

IN(MAX)f

22V(300kHz)

= 303ns

The RSENSE resistor value can be calculated by using the

maximum current sense voltage speciﬁcation with some

accommodation for tolerances:

SENSE ≤

60mV

5.92A

≈ 0.01Ω

Since the output voltage is below 2.4V the output resistive

divider will need to be sized to not only set the output

voltage but also to absorb the SENSE pins speciﬁed input

current.

(MAX) = 24k

0.8V

2.4V – V

OUT

⎛

⎝⎜

⎞

⎠⎟

= 24K

0.8V

2.4V – 1.8V

⎛

⎝⎜

⎞

⎠⎟

= 32k

Choosing 1% resistors; R1 = 25.5k and R2 = 32.4k yields

an output voltage of 1.816V.

The power dissipation on the top side MOSFET can be

easily estimated. Choosing a Siliconix Si4412DY results

in; RDS(ON) = 0.042Ω, CRSS = 100pF. At maximum input

voltage with T(estimated) = 50°C:

MAIN =

22V

()2 1+(0.005)(50°C–25°C)

[]

0.042

()+1.7 22V

()2 5A

() 100pF

() 300kHz

()

= 230mW

A short-circuit to ground will result in a folded back current

of:

SC =

25mV

0.01

200ns(22V)

3.3µH

⎛

⎝⎜

⎞

⎠⎟

= 3.2A

with a typical value of RDS(ON) and δ = (0.005/°C)(20) =

0.1. The resulting power dissipated in the bottom MOSFET

is:

SYNC =

22V – 2V

22V

3.2A

()2 1.1

() 0.042Ω

()

= 430mW

which is less than under full-load conditions.

CIN is chosen for an RMS current rating of at least 3A at

temperature assuming only this channel is on. COUT is

chosen with an ESR of 0.02

Ω for low output ripple. The

output ripple in continuous mode will be highest at the

maximum input voltage. The output voltage ripple due to

ESR is approximately:

VORIPPLE = RESR(ΔIL) = 0.02Ω(1.67A) = 33mVP–P

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