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LTC3890-2

38902f

APPLICATIONS INFORMATION

Design Example

As a design example for one channel, assume VIN = 12V

(nominal), VIN = 22V (max), VOUT = 3.3V, IMAX = 5A,

VSENSE(MAX) = 75mV and f = 350kHz.

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 FREQ pin

to GND, generating 350kHz operation. The minimum

inductance for 30% ripple current is:

ΔI

L =

VOUT

() L()

1–

VOUT

VIN(NOM)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

A 4.7μH inductor will produce 29% ripple current. The

peak inductor current will be the maximum DC value plus

one half the ripple current, or 5.73A. Increasing the ripple

current will also help ensure that the minimum on-time

of 95ns is not violated. The minimum on-time occurs at

maximum VIN:

tON(MIN) =

VOUT

VIN(MAX) f

()

3.3V

22V 350kHz

()

= 429ns

The equivalent RSENSE resistor value can be calculated by

using the minimum value for the maximum current sense

threshold (64mV):

RSENSE ≤

64mV

5.73A

≈ 0.01

Choosing 1% resistors: RA = 25k and RB = 78.7k yields

an output voltage of 3.32V.

The power dissipation on the topside MOSFET can be easily

estimated. Choosing a Fairchild FDS6982S dual MOSFET

results in: RDS(ON) = 0.035Ω/0.022Ω, CMILLER = 215pF. At

maximum input voltage with T(estimated) = 50°C:

PMAIN =

3.3V

22V

()2 1+ 0.005

() 50°C– 25°C

()

⎡⎣

⎤⎦

0.035

()+ 22V

()2

2.5

() 215pF

() •

5V – 2.3V

2.3V

⎡

⎣⎢

⎤

⎦⎥

350kHz

() = 331mW

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

rent of:

ISC =

34mV

0.01

–

95ns 22V

()

4.7μH

⎛

⎝

⎜

⎞

⎠

⎟ = 3.18A

with a typical value of RDS(ON) and δ = (0.005/°C)(25°C)

= 0.125. The resulting power dissipated in the bottom

MOSFET is:

PSYNC = 3.28A

()2 1.125

() 0.022Ω

()

= 250mW

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 0.02Ω 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(1.45A) = 29mVP-P

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