16

 2006 Semtech Corp.

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SC1486A

POWER MANAGEMENT

Application Information (Cont.)

Design Procedure (Cont.)

()

Ohms

10

10

4

.

1

R

2

.

1

I

R

6

)

ON

(

DS

VALLEY

ILIM

−

•

•

•

•

=

For our DDR2 VDDQ example:

I

We select the next lowest 1% resistor value: 13.3k

Ω

Thermal Considerations

The junction temperature of the device may be calculated as follows:

C

P

T

T

JA

D

A

J

°

θ

•

+

=

Where:

T

P

θ

The power dissipation may be calculated as follows:

f

Q

V

I

VCCA

2

P

g

g

VCCA

D

•

•

+

•

•

=

Where:

VCCA = chip supply voltage (V)

I

V

Q

f = switching frequency (kHz)

Inserting the following values as an example:

T

θ

VCCA = 5V

I

V

Q

f = 300kHz (enter the higher of the two set frequencies here)

gives us:

()

C

98

70

10

300

10

60

5

10

1100

5

2

85

T

3

9

6

J

°

=

•

•

•

•

•

+

•

•

•

+

=

−

−

As can be seen, the heating effects due to internal power dissipation are minor, thus requiring no special consider-

ation thermally during layout.