LTC4449 datasheet(31/38 Pages) LINER

31 / 38 page

LTC3774

3774fc

For more information www.linear.com/LTC3774

8. Are the signal and power grounds kept separate? The

IC ground pin and the ground return of CINTVCC must

return to the combined COUT (–) terminals. The VOSNS+

and ITH traces should be as short as possible. The path

formed by the top N-channel MOSFET, Schottky diode

and the CIN capacitor should have short leads and PC

tracelengths.Theoutputcapacitor(–)terminalsshould

be connected as close as possible to the (–) terminals

of the input capacitor by placing the capacitors next to

each other and away from the Schottky loop described

above.

9. Use a modified “star ground” technique: a low imped-

ance, large copper area central grounding point on

the same side of the PC board as the input and output

capacitors with tie-ins for the bottom of the INTVCC

decouplingcapacitor,thebottomofthevoltagefeedback

resistive divider and the GND pin of the IC.

Design Example

As a design example of the front page circuit for a two-

channelhighcurrentregulator,assumeVIN=12V(nominal),

VIN = 20V(maximum), VOUT = 1.5V, IMAX = 60A, and

f = 400kHz (see front page schematic).

The regulated output voltage is determined by:

VOUT = 0.6V • 1+

⎛

⎝

⎜

⎞

⎠

⎟

Using a 10k 1% resistor from the VFB node to ground, the

top feedback resistor is 15k.

The frequency is set by biasing the FREQ pin to 0.75V

(see Figure 12).

The inductance value is based on a 35% maximum ripple

current assumption (10.5A per phase). The highest value

of ripple current occurs at the maximum input voltage:

L =

VOUT

f • ∆IL(MAX)

1−

VOUT

VIN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

This design will require 0.33µH. The Würth 744301033,

0.32µH inductor is chosen. At the nominal input voltage

(12V), the ripple current will be:

∆IL(NOM) =

VOUT

f • L

1−

VOUT

VIN(NOM)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

It will have 10A (33%) ripple. The peak inductor current

will be the maximum DC value plus one-half the ripple

current, or 35A per phase.

The minimum on-time occurs at the maximum VIN, and

should not be less than 100ns (includes margin):

tON(MIN) =

VOUT

VIN(MAX)f

1.5V

20V(400kHz)

= 187ns

DCR sensing is used in this circuit. If C1 and C2 are chosen

to be 220nF, based on the chosen 0.33µH inductor with

0.32mΩ DCR, R1 and R2 can be calculated as:

R1=

DCR • C1

= 4.69k

R2 =

DCR • C2 • 5

= 937Ω

Choose R1 = 4.64k and R2 = 931Ω.

APPLICATIONS INFORMATION

LTC4449 Datenblatt(PDF) 31 Page - Linear Technology

LTC4449 Datenblatt(HTML) 31 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 33 34 35 36 37 38

Datenblatt Download

Link URL