6

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OPA541

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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. 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)

– 0.057

(AP)

R

– 0.02

0.809

0.813

Because of the internal structure of the OPA541, the actual

current limit depends on whether current is positive or

negative. The above R

expected level, while –I

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

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.

selected for sufficient power dissipation. For a 5A current

0.105

Ω (0.143Ω on the power plastic package due to differ-

ent internal resistances). A continuous 5A through 0.105

Ω

would require an R

∆R

x (T – 25)

–2mV

|I

Sinusoidal outputs create dissipation according to rms load

current. For the same R

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

excessive inductance and may cause loop-stability prob-

lems. Be sure to evaluate circuit performance with resistor

type planned for production to assure proper circuit opera-

tion.

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 Burr-

Brown Application Bulletin AB-038 for further details on

heat sinking. On the power plastic package, the metal tab is

when mounting on a heat sink or chassis.

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

across the output transistor decreases and the device can

safely provide large output currents demanded by the load.