Datasheet
SLVS167C − SEPTEMBER 1998 − REVISED − MAY 1999
20
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
thermal considerations
DISSIPATION RATING TABLE
PACKAGE
AIR FLOW
(CFM)
T
A
< 25°C
POWER RATING
DERATING FACTOR
ABOVE T
A
= 25°C
T
A
= 70°C
POWER RATING
T
A
= 85°C
POWER RATING
PWP
‡
0 2.9 W 23.5 mW/°C 1.9 W 1.5 W
PWP
‡
300 4.3 W 34.6 mW/°C 2.8 W 2.2 W
PWP
§
0 3 W 23.8 mW/°C 1.9 W 1.5 W
PWP
§
300 7.2 W 57.9 mW/°C 4.6 W 3.8 W
†
This parameter is measured with the recommended copper heat sink pattern on a 1-layer PCB, 5-in × 5-in PCB,
1 oz. copper, 2-in × 2-in coverage (4 in
2
).
‡
This parameter is measured with the recommended copper heat sink pattern on an 8-layer PCB, 1.5-in × 2-in PCB, 1 oz. copper
with layers 1, 2, 4, 5, 7, and 8 at 5% coverage (0.9 in
2
) and layers 3 and 6 at 100% coverage (6 in
2
).
The maximum ambient temperature depends on the heatsinking ability of the PCB system. Using the 0 CFM
and 300 CFM data from the dissipation rating table, the derating factor for the PWP package with 6.9 in
2
of
copper area on a multilayer PCB is 24 mW/°C and 58 mW/°C respectively. Converting this to Θ
JA
:
Θ
JA
+
1
Derating
+
1
0.0235
+ 42.6°CńW
+
1
0.0579
+ 17.3°CńW
For 0 CFM : For 300 CFM :
Given Θ
JA
, the maximum allowable junction temperature, and the total internal dissipation, the maximum
ambient temperature can be calculated with the following equation. The maximum recommended junction
temperature for the TPS73HD3xx is 150 °C.
T
A
Max + T
J
Max *
ǒ
Θ
JA
P
D
Ǔ
The maximum power dissipation limit is determined using the following equation:
T
D(max)
+
T
J
max * T
A
R
θJA
Where:
T
J
max is the maximum allowable junction temperature
R
θJA
is the thermal resistance junction-to-free-air for the package (i.e., 285°C/W for the 5-terminal
SOT-23 package.
T
A
is the free-air temperautre
The regulator dissipation is calculated using:
P
D
+
ǒ
V
I
* V
O
Ǔ
1
O
Power dissipation resulting from quiescent current is negligible.