Philips Semiconductors

Product data

NE57811

Advanced DDR memory termination power

with shutdown

2003 Apr 02

7

THERMAL DESIGN

Designing the proper thermal system for the NE57811 is important

to its reliable operation. The NE57811 will be operating at an

average power level less than the maximum rating of the part. In a

typical DDR terminator system the average power dissipation is

between 0.8 and 1.5 watts. The termination power will vary as the

average number of ‘1s’ and ‘0s’ changes during normal operation of

the DDR memory. The load current will assume a new value for

each bus cycle at a 266 MHz rate, and will increase and decrease

as the statistical average of bus states change.

The terminator heatsink must be designed to accommodate the

average power as a steady state condition and be able to withstand

momentary periods of increased dissipation, typically 2 – 5 seconds

duration. For the typical NE57811 application, the power dissipated

by the terminator can be calculated:

P

Eqn. (1)

The thermal resistance of a surface mount package is given as

specifies a 4-layer multiplayer PCB (2oz/1oz/1oz/2oz copper) that is

4 inches on each side. This is probably the best (or lowest) thermal

resistance you will see in any application. Most applications cannot

afford the PCB area to create this situation, but the thermal

performance of a multilayer PCB will still provide a significant

heatsinking effect. The actual thermal resistance will be higher than

the 16.5

°C/W given for the 4-layer JEDEC PCB.

Figure 8 shows the thermal resistance you can expect for

heatsinking PCB areas less than the JEDEC specification. The

graph is for a 2 oz. single-sided PCB with a square area of the side

dimension as given on the X axis. If you use a double-sided PCB

with some plated-through holes to help transfer heat to the bottom

side, the thermal resistance only improves by about 3 – 4

°C/W.

After the power is estimated, the minimum PCB area can be

determined by calculating the worst case thermal resistance and

referring to Figure 8 to determine the PCB area. This is done by:

R

T

P

D

Eqn. (2)

Where:

The junction temperature should be kept well away from the

over-temperature cutoff threshold temperature (+150

°C) in normal

operation.

Using the above power dissipation, the highest ambient temperature

and a junction temperature of +125

°C, calculate the maximum

thermal resistance (1.5 watts is used only as an example).

R

C

1.5

W

Eqn. (3)

Looking at Figure 8, you see that this power dissipation requires a

is the smallest area you could use at this power dissipation. Of

course, increasing this area will allow the NE57811 to operate at

cooler temperatures, thus enhancing its long-term reliability.

SL01670

LENGTH OF SIDE OF 2 oz. COPPER AREA (mm)

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

0

20

40

60

80

100

Figure 8.

PCB heatsink area versus thermal resistance.

SL01678

0.1

1

10

12

3

4

5

6

7

8

9 10

0.25 s

0.5 s

DC

Figure 9.

Safe operating area for the NE57811.