IR3502

Page 29 of 39

July 28, 2009

The threshold voltage of VRHOT comparator is fixed at 1.6V, and a negative temperature coefficient (NTC)

)]

1

1

(

*

[

*

_

_

ROOM

MAX

L

THERM

THERM

TMAX

T

T

B

EXP

R

R

−

=

(27)

TMAX from (28).

6

.

1

)

6

.

1

(

*

)

(

2

1

−

+

=

VCCL

R

R

R

HOTSET

TMAX

HOTSET

(28)

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 Intel VR11 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.

[

mA

f

Q

Q

I

sw

gt

gb

avg

drive

⋅

+

⋅

+

=

10

)

(

_

(29)

min

_

/

8

.

6

7

.

0

(min)

β

avg

drive

I

VCCLDRV

I

V

V

R

−

−

=

(30)

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

mA

R

V

V

VCCLDRV

I

10

8

.

6

7

.

0

(max)

<

−

−

(31)

be used instead of a single NPN transistor.

Current Monitor Filter

A filter is added to isolate the CPU from rapid changes in the load current and trigger false response. A filter with

300 us time constant provides adequate delay for Intel VR11.1 response. A 1k resistor between IMON and local

ground helps equalize the source and sink current of the IMON pin.