IR3513Z

Page 28

May 11, 2009

Since VCCL voltage is proportional to the MOSFET gate driver loss and inversely proportional to the MOSFET

conduction loss, the optimum voltage should be chosen to maximize the converter efficiency. VCCL linear

regulator consists of an external NPN transistor, a ceramic capacitor and a programmable resistor divider. Pre-

19

.

1

19

.

1

*

1

2

−

=

VCCL

R

R

VCCLFB

VCCLFB

(11)

The capacitor is selected based on the stability requirement of the linear regulator and the load current to be driven.

The linear regulator supplies the bias and gate drive current of the phase ICs. A 4.7uF normally ensures stable VCCL

performance for most applications.

The drive resistor is primarily dependent on the load current requirement of the linear regulator and the minimum

input voltage requirements. The following equation gives an estimate of the average load current of the switching

phase ICs.

C

VCCL

PHS

VCCL

gt

gb

avg

drive

I

n

I

n

Q

Q

I

_

_

_

)

1

(

)

(

+

−

⋅

+

⋅

+

=

(12)

min

_

/

(max)

7

.

0

(min)

β

avg

drive

I

VCCLDRV

I

VCCL

V

R

−

−

=

(13)

Due to limited pull down capability of the VCCLDRV pin, make sure the following condition is satisfied.

mA

R

VCCL

V

VCCLDRV

I

10

(min)

7

.

0

(max)

<

−

−

(14)

instead of a single NPN transistor.

in parallel with the inductor are chosen to match the time constant of the inductor, and therefore the voltage across

the capacitor voltage is different from that of the real inductor current. The time constant mismatch does not affect the

average current sharing among the multiple phases, but affects the current signal ISHARE.

follows.

CS

L

CS

C

R

L

R

=

(15)