OPA561PWPSLASH2K datasheet(9/16 Pages) TI

9 / 16 page

OPA561

SBOS206D − DECEMBER 2001 − REVISED OCTOBER 2004

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OUTPUT PROTECTION

Reactive and EMF-generation 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. Schottky rectifier diodes with a 3A or greater

continuous rating are recommended.

Figure 4. Output Protection Diode

THERMAL PROTECTION

The OPA561 has thermal sensing circuitry that helps

protect the amplifier from exceeding temperature limits.

Power dissipated in the OPA561 will cause the junction

temperature to rise. Internal thermal shutdown circuitry

shuts down the output when the die temperature reaches

approximately 160

°C, resetting when the die has cooled to

140

°C. Depending on load and signal conditions, the

thermal protection circuit may cycle on and off. This limits

the dissipation 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 heatsink. For reliable,

long-term, continuous operation, junction temperature

should be limited to +125

°C, maximum. To estimate the

margin of safety in a complete design (including heatsink),

increase the ambient temperature until the thermal

protection is triggered. Use worst-case loading and signal

conditions.

For

good,

long-term

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 OPA561 was

designed to protect against overload conditions; it was not

intended to replace proper heatsinking. Continuously

running the OPA561 into thermal shutdown can degrade

reliability. The E/S pin can be monitored to determine if

shutdown has occurred.

During normal operation the

voltage on the E/S pin is typically above (V−) + 2V. During

shutdown, the voltage drops to less than (V−) + 0.8V.

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 conducting output transistor. Dissipation with ac

signals is lower. Application Bulletin AB−039 (SBOA022)

explains how to calculate or measure power dissipation

with unusual signals and loads, and can be dowloaded

from www.ti.com.

HEATSINK AREA

The relationship between thermal resistance and power

dissipation can be expressed as:

where:

TJ = Junction Temperature (°C)

TA = Ambient Temperature (°C)

qJA = Junction-to-Ambient Thermal Resistance (°C/W)

PD = Power Dissipation (W)

To appropriately determine required heatsink area,

required power dissipation should be calculated and the

relationship between power dissipation and thermal

resistance should be considered to minimize shutdown

conditions and allow for proper long-term operation

(junction temperature of +125

°C). Once the heatsink area

has been selected, worst-case load conditions should be

tested to ensure proper thermal protection.

For applications with limited board size, refer to Figure 5

for the approximate thermal resistance relative to heatsink

area. Increasing heatsink area beyond 2in2 provides little

improvement in thermal resistance. To achieve the

°C/W stated in the Electrical Characteristics, a copper

plane size of 9in2 was used. The HTSSOP-20 PowerPAD

package is well suited for continuous power levels from

2W to 4W, depending on ambient temperature and

heatsink area. Higher power levels may be achieved in

applications with a low on/off duty cycle, such as remote

meter reading.

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