THERMAL RESISTANCE
vs ALUMINUM PLATE AREA
18
Aluminum Plate Area
Vertically Mounted
in Free Air
16
Flat, Rectangular
Aluminum Plate
Thermal Resistance
θ
JA
(°C/W)
14
0.030in Al
12
0.050in Al
10
Aluminum
Plate Thickness
8
0
1
2
3
4
5
6
7
8
Aluminum Plate Area (inches
2
)
0.062in Al
Optional mica or film insulator
for electrical isolation. Adds
OPA548
approximately 1°C/W.
TO-220 Package
FIGURE 5. TO-220 Thermal Resistance vs Aluminum Plate Area.
THERMAL RESISTANCE vs
CIRCUIT BOARD COPPER AREA
50
Circuit Board Copper Area
Thermal Resistance,
θ
JA
(°C/W)
40
OPA548F
Surface Mount Package
1oz copper
30
20
10
0
0
1
2
3
4
5
Copper Area (inches
2
)
OPA548
Surface Mount Package
FIGURE 6. DDPAK Thermal Resistance vs Circuit Board Copper Area.
conducting output transistor. Power dissipation can be mini-
mized by using the lowest possible power supply voltage
necessary to assure the required output voltage swing.
For resistive loads, the maximum power dissipation occurs
at a dc output voltage of one-half the power supply voltage.
Dissipation with ac signals is lower. Application Bulletin
AB-039 explains how to calculate or measure power dissi-
pation with unusual signals and loads.
THERMAL PROTECTION
Power dissipated in the OPA548 will cause the junction
temperature to rise. The OPA548 has thermal shutdown
circuitry that protects the amplifier from damage. The ther-
mal protection circuitry disables the output when the junc-
tion temperature reaches approximately 160°C, allowing the
device to cool. When the junction temperature cools to
approximately 140°C, the output circuitry is again enabled.
Depending on load and signal conditions, the thermal pro-
tection circuit may cycle on and off. This limits the dissipa-
tion of the amplifier but may have an undesirable effect on
the load.
Any tendency to activate the thermal protection circuit
indicates excessive power dissipation or an inadequate heat
sink. For reliable operation, junction temperature should be
limited to 125°C, maximum. To estimate the margin of
safety in a complete design (including heat sink) increase the
ambient temperature until the thermal protection is trig-
gered. Use worst-case load and signal conditions. For good
reliability, thermal protection should trigger more than 35°C
above the maximum expected ambient condition of your
application. This produces a junction temperature of 125°C
at the maximum expected ambient condition.
The internal protection circuitry of the OPA548 was de-
signed to protect against overload conditions. It was not
intended to replace proper heat sinking. Continuously run-
ning the OPA548 into thermal shutdown will degrade reli-
ability.
®
OPA548
10
BURR-BROWN [ BURR-BROWN CORPORATION ]
