7.3.3 Thermal Protection

The internal power dissipation of any amplifier causes its internal (junction) temperature to rise. This

phenomenon is called self heating. The absolute maximum junction temperature of the OPAx992-Q1 is 150°C.

Exceeding this temperature causes damage to the device. The OPAx992-Q1 has a thermal protection feature

that reduces damage from self heating. The protection works by monitoring the temperature of the device and

turning off the op amp output drive for temperatures above 170°C. Figure 7-4 shows an application example for

the OPA2992-Q1 that has significant self heating because of its power dissipation (0.954 W). In this example,

both channels have a quiescent power dissipation while one of the channels has a significant load. Thermal

calculations indicate that for an ambient temperature of 55°C, the device junction temperature reaches 180°C.

The actual device, however, turns off the output drive to recover towards a safe junction temperature. Figure 7-4

shows how the circuit behaves during thermal protection. During normal operation, the device acts as a buffer so

the output is 3 V. When self heating causes the device junction temperature to increase above the internal limit,

the thermal protection forces the output to a high-impedance state and the output is pulled to ground through

resistor RL. If the condition that caused excessive power dissipation is not removed, the amplifier will oscillate

between a shutdown and enabled state until the output fault is corrected. Please note that thermal performance

can vary greatly depending on the package selected and the PCB layout design. This example uses the thermal

performance of the SOIC (8) package.

170ºC

3 V

0 V

30 V

RL

100

VIN

3 V

+

3 V

–

I

OUT = 30 mA

OPA2992

One channel has load

Consider IQ of two channels

TA = 55°C

PD = 0.954W

JA = 131°C/W

TJ = 131°C/W × 0.954W + 55°C

TJ = 180°C (expected)

+

–

Figure 7-4. Thermal Protection

7.3.4 Capacitive Load and Stability

The OPAx992-Q1 features an output stage capable of driving moderate capacitive loads, and by leveraging

an isolation resistor, the device can easily be configured to drive larger capacitive loads. Increasing the gain

enhances the ability of the amplifier to drive greater capacitive loads; see Figure 7-5 and Figure 7-6. The

particular op amp circuit configuration, layout, gain, and output loading are some of the factors to consider when

establishing whether an amplifier will be stable in operation.

Capacitive Load (pF)

0

80

160

240

320

400

480

560

0

10

20

30

40

50

60

70

D030

RISO = 0:, Overshoot (+)

RISO = 0:, Overshoot (-)

RISO = 50:, Overshoot (+)

RISO = 50:, Overshoot (-)

Figure 7-5. Small-Signal Overshoot vs Capacitive

Load (20-mVpp Output Step, G = +1)

Capacitive Load (pF)

0

80

160

240

320

400

480

560

0

10

20

30

40

50

60

70

D029

RISO = 0:, Overshoot (+)

RISO = 0:, Overshoot (-)

RISO = 50:, Overshoot (+)

RISO = 50:, Overshoot (-)

Figure 7-6. Small-Signal Overshoot vs Capacitive

Load (20-mVpp Output Step, G = -1)

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