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 OPAx990-Q1 is 150°C.

Exceeding this temperature causes damage to the device. The OPAx990-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 OPA990-Q1 that has significant self heating because of its power dissipation (0.81 W). Thermal calculations

indicate that for an ambient temperature of 65°C, the device junction temperature must reach 177°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, then the amplifier will oscillate between a

shutdown and enabled state until the output fault is corrected.

170ºC

3 V

0 V

30 V

RL

100

VIN

3 V

+

3 V

–

I

OUT = 30 mA

OPA990-Q1

TA = 65°C

PD = 0.81W

JA = 138.7°C/W

TJ = 138.7°C/W × 0.81W + 65°C

TJ = 177.3°C (expected)

+

–

Figure 7-4. Thermal Protection

7.3.4 Capacitive Load and Stability

The OPAx990-Q1 features a resistive output stage capable of driving moderate capacitive loads, and by

leveraging an isolation resistor, the device can easily be configured to drive large capacitive loads. As shown in

Figure 7-5 and Figure 7-6, increasing the gain enhances the ability of the amplifier to drive greater capacitive

loads. 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.

Cap Load (pF)

0

40

80

120

160

200

240

280

320

360

5

10

15

20

25

30

35

40

45

50

55

C008

RISO = 0 :, Positive Overshoot

RISO = 0 :, Negative Overshoot

RISO = 50 :, Positive Overshoot

RISO = 50 :, Negative Overshoot

Figure 7-5. Small-Signal Overshoot vs Capacitive

Load (10-mV Output Step, G = 1)

Cap Load (pF)

0

40

80

120

160

200

240

280

320

360

3

6

9

12

15

18

21

24

27

30

33

C007

RISO = 0 :, Positive Overshoot

RISO = 0 :, Negative Overshoot

RISO = 50 :, Positive Overshoot

RISO = 50 :, Negative Overshoot

Figure 7-6. Small-Signal Overshoot vs Capacitive

Load (10-mV Output Step, G = –1)

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OPA4990-Q1

SBOSAH7 – OCTOBER 2023

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