NE57810S Philipss, NE57810S Datasheet - Page 8

• Manufacturer Part Number: NE57810S
• Description: Advanced DDR memory termination power with external reference in
• Manufacturer: Philipss

Philips Semiconductors
THERMAL DESIGN
Designing the proper thermal system for the NE57810 is important
to its reliable operation. The NE57810 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 NE57810 application, the power dissipated
by the terminator can be calculated:
The thermal resistance of a surface mount package is given as
R
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 10 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
2003 Sep 12
th(j-a
Advanced DDR memory termination power
with external reference in
Figure 10. PCB heatsink area versus thermal resistance.
P
), the thermal resistance from the junction to air. JESD51-7
40.0
35.0
30.0
25.0
20.0
15.0
10.0
D
5.0
0.0
0
I
DD ( VTT )
LENGTH OF SIDE OF 2 oz. COPPER AREA (mm)
Watts
20
40
60
80
Eqn. (1)
SL01670
100
8
Using the above power dissipation, the highest ambient temperature
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 10 to determine the PCB area. This is done by:
Where:
The junction temperature should be kept well away from the
over-temperature cutoff threshold temperature (+150 C) in normal
operation.
and a junction temperature of +125 C, calculate the maximum
thermal resistance (1.5 watts is used only as an example).
Looking at Figure 10, you see that this power dissipation requires a
minimum PCB island area of 225 mm
is the smallest area you could use at this power dissipation. Of
course, increasing this area will allow the NE57810 to operate at
cooler temperatures, thus enhancing its long-term reliability.
T
T
P
amb
j
D
R
R
is the maximum desired junction temperature
is the estimated average power
qJA ( min )
th ( j–a )( min )
0.1
10
Figure 11. Safe operating area for the NE57810.
is the highest expected local ambient temperature
1
1
T
j
125
P
T
D
O
amb
C
1.5 W
2
70
O
V
C
DD
3
2
(V)
(15 mm on each side). This
36.6
0.25 s
0.5 s
DC
4
O
C W
5
NE57810
6
Product data
7
Eqn. (2)
Eqn. (3)
SL01678
8 9 10