LTC1735 datasheet(25/32 Pages) LINER

25 / 32 page

LTC3728L-1

3728l1fc

APPLICATIONS INFORMATION

Design Example

As a design example for one channel, assume VIN =

12V(nominal), VIN = 22V(max), VOUT = 1.8V, 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 PLLFLTR

pin to a resistive divider from the INTVCC pin, generating

0.7V for 300kHz operation. The minimum inductance for

30% ripple current is:

VIN –VOUT

(f)(IRIPPLE)

•

VOUT

VIN

or 3.7μH. Using standard inductor values:

IL =

VOUT

(f)(L)

1–

VOUT

VIN

A 4.7μH inductor will produce 23% ripple current and a

3.3μH will result in 33%. The peak inductor current will be

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

5.84A, for the 3.3μH value. Increasing the ripple current

will also help ensure that the minimum on-time of 100ns

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

VIN:

tON(MIN) =

VOUT

VIN(MAX)f

1.8V

22V(300kHz)

= 273ns

The RSENSE resistor value can be calculated by using the

maximum current sense voltage speciﬁcation with some

accommodation for tolerances:

RSENSE

60mV

5.84A

0.01

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 pin’s speciﬁed

input current.

R1(MAX) = 24k

0.8V

2.4V – VOUT

= 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 Fairchild FDS6982S dual MOSFET

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

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

PMAIN =

1.8V

22V

()2 1+(0.005)(50°C–25°C)

[]•

0.035

()+ 22V

()2

() 215pF

()•

5– 2.3

2.3

300kHz

()=332mW

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

current of:

ISC =

25mV

0.01

–

120ns(22V)

3.3

μH

= 2.1A

with a typical value of RDS(ON)and δ = (0.005/°C)(20) = 0.1.

The resulting power dissipated in the bottom MOSFET is:

PSYNC =

22V – 1.8V

22V

2.1A

()2 1.125

() 0.022

()

= 100mW

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

LTC1735 Datasheet(PDF) 25 Page - Linear Technology

LTC1735 Datasheet(HTML) 25 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL