LTC4449 datasheet(44/48 Pages) AD

44 / 48 page

LTC7872

Rev. 0

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(30A/phase), and f = 150kHz. The regulated output voltage

is determined by:

VLOW = 1.2V • (1 + RB/RA).

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

the top feedback resistor is (to the nearest 1% standard

value) 90.9k. The frequency is set by selecting the RFREQ

to be 37.4kΩ. The inductance values are based on a 35%

maximum ripple current assumption (10.5A for each

phase). The highest value of ripple current occurs at the

maximum VHIGH voltage:

VLOW

f • ∆IL MAX

(

)

• 1–

VLOW

VHIGH MAX

(

)

⎛

⎝⎜

⎞

⎠⎟

Each phase will require 6.1μH. The Sagami CVE2622C-

6R8M, 6.8μH, 1.8mΩ DCR inductor is chosen. At the

nominal VHIGH voltage (48V), the ripple current will be:

∆IL NOM

(

) =

VLOW

f •L

• 1–

VLOW

VHIGH NOM

(

)

⎛

⎝⎜

⎞

⎠⎟

Each phase will have 8.8A (29.3%) ripple. The peak induc-

tor current will be the maximum DC value plus one-half the

ripple current, or 34.4A. The minimum on-time occurs at

the maximum VHIGH, and should not be less than 150ns:

TON MIN

(

) =

VLOW

VHIGH MAX

(

) • f

12V

60V •150kHz

= 1.33µs

With VILIM = 3/4 VV5, the equivalent RSENSE resistor value

can be calculated by using the minimum value for the

maximum current sense threshold (45mV):

RSENSE EQUIV

(

) =

VSENSE MIN

(

)

ILOAD MAX

(

)

#OF PHASES

∆IL NOM

(

)

The equivalent required RSENSE value is 1.31mΩ. Choose

RS = 1mΩ to allow some design margin. As shown in

Figure 17, set R2 to be below 1/10th of the R1. Therefore,

the DC component of the SNSA+ filter is small enough to

be omitted. R1 • C1 should have a bandwidth that is four

times as high as the L/RS.

Special Layout Consideration

Exceeding Absolute Max ratings on the EXTVCC pin can

result in damage to the controller. As the EXTVCC pin is

normally connected to VLOW, it is recommended to put

a Schottky diode with an appropriately high voltage rat-

ing between the VLOW and the EXTVCC pins as shown in

Figure 16(a). Choose the right Schottky diode with the

forward voltage less than 0.5V at the maximum EXTVCC

pin current.

Another method to protect on the EXTVCC pin is to use a

Schottky diode to clamp the EXTVCC pin to reduce volt-

age spiking below ground. The Schottky diode should be

placed close to the controller IC, with the cathode con-

nected to the EXTVCC pin and the anode connected to

ground as shown in the Figure 16(b). Choose a minimum

1Ω RFLTR and keep the maximum voltage drop across the

RFLTR less than 0.5V.

Figure 16.

LTC7872

RFLTR

1 F

VLOW

EXTVCC

(a)

LTC7872

VLOW

EXTVCC

1 F

(b)

7872 F16

Methods to Protect the EXTVCC Pin

Design Example

As a design example for a four-phase single output

high current regulator, assume VHIGH = 48V (nominal),

VHIGH = 60V (maximum), VLOW = 12V, IVLOW(MAX) = 120A

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