OPA547: The OPA547 heats even when there is no load

Hello Pierre,

If the OPA547 DDPAK package is not soldered to a heatsink surface, then the thermal resistance from the op amp junctions to the air will be the θ_JA = 65°C/W listed in the datasheet Electrical Characteristics table. Since the semiconductor die internal to the OPA547 is attached directly to the metal DDPAK inner surface, it and the plastic material after several thermal time constants will eventually assume a temperature a little less than the actual Tj temperature.

Here are two scenarios based on the typical quiescent current of 10 mA, and the maximum quiescent current of 15 mA:

If Iq = 0.01 Amps typical, and (V+) - (V-) = 30 V

T_J = T_A + θ_JA ∙ Pd

T_J = 25°C + 65°C/W [(30 V)(0.01A)]

T_J = 25°C + (65°C/W ∙ 0.3 W)

T_J = 44.5°C

If Iq = 0.015 Amps maximum, and (V+) - (V-) = 30 V

T_J = 25°C + 65°C/W [(30 V)(0.015A)]

T_J = 25°C + (65°C/W ∙ 0.45 W)

T_J = 54.3°C

You will note that in the second, maximum case the Tj ≈ 55°C. If the OPA547 is left powered "on" long enough the case will approach the same temperature. The OPA547 maximum specified Tj = 150°C; therefore, there isn't any issue with running the device with a Tj ≈ 55°C. 

Note that the OPA547 DDPAK solder tab be electrically connected to the V- supply.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi Thomas,

Thank you very much for your detailed calculations.

Everything is explained now.

Pierre

Hello Pierre,

I am glad to be of assistance with your OPA547 inquiry.

Regards, Thomas

Precision Amplifiers Applications Engineerig