Datasheet

OPA541
6
SBOS153A
www.ti.com
θ
HS
=
T
CASE
– T
AMBIENT
P
D
(max)
INSTALLATION
INSTRUCTIONS
POWER SUPPLIES
The OPA541 is specified for operation from power supplies
up to ±40V. It can also be operated from unbalanced power
supplies or a single power supply, as long as the total power
supply voltage does not exceed 80V. The power supplies
should be bypassed with low series impedance capacitors
such as ceramic or tantalum. These should be located as near
as practical to the amplifier’s power supply pins. Good
power amplifier circuit layout is, in general, like good high
frequency layout. Consider the path of large power supply
and output currents. Avoid routing these connections near
low-level input circuitry to avoid waveform distortion and
oscillations.
CURRENT LIMIT
Internal current limit circuitry is controlled by a single
external resistor, R
CL
. Output load current flows through this
external resistor. The current limit is activated when the
voltage across this resistor is approximately a base-emitter
turn-on voltage. The value of the current limit resistor is
approximately:
(AM, BM, SM) R
CL
= – 0.057
(AP) R
CL
= – 0.02
0.809
|I
LIM
|
0.813
|I
LIM
|
Because of the internal structure of the OPA541, the actual
current limit depends on whether current is positive or
negative. The above R
CL
gives an average value. For a given
R
CL
, +I
OUT
will actually be limited at about 10% below the
expected level, while –I
OUT
will be limited about 10% above
the expected level.
The current limit value decreases with increasing tempera-
ture due to the temperature coefficient of a base-emitter
junction voltage. Similarly, the current limit value increases
at low temperatures. Current limit versus resistor value and
temperature effects are shown in the Typical Performance
Curves. Approximate values for R
CL
at other temperatures
may be calculated by adjusting R
CL
as follows:
The adjustable current limit can be set to provide protection
from short circuits. The safe short-circuit current depends on
power supply voltage. See the discussion on Safe Operating
Area to determine the proper current limit value.
Since the full load current flows through R
CL
, it must be
selected for sufficient power dissipation. For a 5A current
limit on the TO-3 package, the formula yields an R
CL
of
0.105Ω (0.143Ω on the power plastic package due to differ-
ent internal resistances). A continuous 5A through 0.105Ω
would require an R
CL
that can dissipate 2.625W.
∆R
CL
= x (T – 25)
–2mV
|I
LIM
|
Sinusoidal outputs create dissipation according to rms load
current. For the same R
CL
, AC peaks would still be limited
to 5A, but rms current would be 3.5A, and a current limiting
resistor with a lower power rating could be used. Some
applications (such as voice amplification) are assured of
signals with much lower duty cycles, allowing a current
resistor with a low power rating. Wire-wound resistors may
be used for R
CL
. Some wire-wound resistors, however, have
excessive inductance and may cause loop-stability prob-
lems. Be sure to evaluate circuit performance with the
resistor type planned for production to assure proper circuit
operation.
HEAT SINKING
Power amplifiers are rated by case temperature, not ambient
temperature as with signal op amps. Sufficient heat sinking
must be provided to keep the case temperature within rated
limits for the maximum ambient temperature and power
dissipation. The thermal resistance of the heat sink required
may be calculated by:
Commercially available heat sinks often specify their ther-
mal resistance. These ratings are often suspect, however,
since they depend greatly on the mounting environment and
air flow conditions. Actual thermal performance should be
verified by measurement of case temperature under the
required load and environmental conditions.
No insulating hardware is required when using the TO-3
package. Since mica and other similar insulators typically
add approximately 0.7°C/W thermal resistance, their elimi-
nation significantly improves thermal performance. See
Application Bulletin SBOA021, available for download at
www.ti.com, for further details on heat sinking. On the
power plastic package, the metal tab may have a high or low
impedance connection to –V
S
. The case should be allowed to
float and will likely assume the potential of –V
S
. Current
should NOT be conducted through the case.
SAFE OPERATING AREA
The safe operating area (SOA) plot provides comprehensive
information on the power handling abilities of the OPA541.
It shows the allowable output current as a function of the
voltage across the conducting output transistor (see Figure
1). This voltage is equal to the power supply voltage minus
the output voltage. For example, as the amplifier output
swings near the positive power supply voltage, the voltage