POWER DISSIPATION
Power dissipation depends on power supply, signal and load
conditions. For dc signals, power dissipation is equal to the
product of output current times the voltage across the con-
ducting 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.
HEATSINKING
Most applications require a heat sink to assure that the
maximum junction temperature is not exceeded. The heat
sink required depends on the power dissipated and on
ambient conditions. Consult Application Bulletin AB-038
for information on determining heat sink requirements.
The mounting tab of the surface-mount package version
should be soldered to a circuit board copper area for good
heat dissipation. Figure 3 shows typical thermal resistance
from junction to ambient as a function of the copper area.
THERMAL PROTECTION
The OPA544 has thermal shutdown that protects the ampli-
fier from damage. Any tendency to activate the thermal
shutdown circuit during normal operation is indication of
excessive power dissipation or an inadequate heat sink.
The thermal protection activates at a junction temperature of
approximately 155˚C. For reliable operation, junction tem-
perature should be limited to 150˚C, maximum. To estimate
the margin of safety in a complete design (including heat
sink), increase the ambient temperature until the thermal
protection is activated. Use worst-case load and signal con-
ditions. For good reliability, the thermal protection should
trigger more than 25˚C above the maximum expected ambi-
ent condition of your application. This produces a junction
temperature of 125˚C at the maximum expected ambient
condition.
THERMAL RESISTANCE vs
CIRCUIT BOARD COPPER AREA
50
Thermal Resistance,
θ
JA
(°C/W)
Depending on load and signal conditions, the thermal pro-
tection circuit may produce a duty-cycle modulated output
signal. This limits the dissipation in the amplifier, but the
rapidly varying output waveform may be damaging to some
loads. The thermal protection may behave differently de-
pending on whether internal dissipation is produced by
sourcing or sinking output current.
OUTPUT STAGE COMPENSATION
The complex load impedances common in power op amp
applications can cause output stage instability. Figure 3
shows an output series R/C compensation network (1Ω in
series with 0.01µF) which generally provides excellent sta-
bility. Some variation in circuit values may be required with
certain loads.
UNBALANCED POWER SUPPLIES
Some applications do not require equal positive and negative
output voltage swing. The power supply voltages of the
OPA544 do not need to be equal. For example, a –6V
negative power supply voltage assures that the inputs of the
OPA544 are operated within their linear common-mode
range, and that the output can swing to 0V. The V+ power
supply could range from 15V to 65V. The total voltage (V–
to V+) can range from 20V to 70V. With a 65V positive
supply voltage, the device may not be protected from dam-
age during short-circuits because of the larger V
CE
during
this condition.
OUTPUT PROTECTION
Reactive and EMF-generating loads can return load current
to the amplifier, causing the output voltage to exceed the
power supply voltage. This damaging condition can be
avoided with clamp diodes from the output terminal to the
power supplies as shown in Figure 4. Fast-recovery rectifier
diodes with a 4A or greater continuous rating are recom-
mended.
Circuit Board Copper Area
40
OPA544F
Surface Mount Package
1oz copper
30
20
10
OPA544
Surface Mount Package
0
0
1
2
Copper Area
3
(inches
2
)
4
5
FIGURE 3. Thermal Resistance vs Circuit Board Copper Area.
®
7
OPA544
TI [ TEXAS INSTRUMENTS ]
