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# OPA564-Q1: θja vs. θjc

Part Number: OPA564-Q1

Hi team,

My customer provided a question in terms of thermal resistance for OPA564-Q1.

Datasheet shows(Pad Down) θjc=50℃/W while θja=33℃/W so  θjc>θja.

Is this correct information?

The customer doubts θjc would be wrong...

I'm looking forward to hearing back from you.

Best regards,

Shota Mago

• Hello Shota,

There appears to be a problem with the PowerPad thermal table for OPA564-Q1 packages. Some of the theta numbers are not correctly specified in the table.

This issue seemed familiar to me and I found a similar e2e forum inquiry from last year that also questioned the PowerPad down theta numbers. I found correct information and provided it in my response to that inquiry. You can find the information here:

I am going to forward this information to our internal group that is responsible for datasheet corrections. We should be able to get corrections made to the OPA546 and OPA546-Q1 datasheet thermal tables.

Regards, Thomas

Precision Amplifiers Applications Engineering

• Hi Thomas-san

Thanks for kind explanation.

But is θjc(top)=50℃/W still correct?
When the customer measured top of OPA564-Q1 with 3W dissipation, it was 87℃.
Does it mean that Tj=85℃+50℃/W*3W=237℃? However this calculation result should be wrong.

While, which point(air) does θja mean?
Is that peripheral temperature at bottom side?
How can we derive junction temperature from OPA564-Q1 datasheet information?

Best regards,
Shota Mago
• Hello Mago-san,

The OPA564-Q1 thermal table information is confusing and we are attempting to understand and resolve them. If you are using the DWP PowerPad down version of the package and the θjc(top) = 50℃/W is correct, that path would not be the primary path for heat conduction. That path to the top would be a the thermal resistance from the junction to the top surface of the package which would rely upon radiation for heat dissipation.

More importantly is the thermal path from the OPA564-Q1 junction, through the thermal pad to its outer surface. That appears to be the θJP 1.83°C/W number in the datasheet table, which is also the θJC(bottom) number listed in the earlier referenced e2e response. That is the primary path by which heat will be conducted away from the OPA564-Q1 junction. But keep in mind that even with that low 1.83°C/W θJC(BOTTOM), that the thermal resistance of the PC board pad to the outside world to which the OPA564-Q1 PowerPad is soldered must be added to the total. That would be the junction to ambient thermal resistance θJA of primary importance.

Using the Figure 47. Thermal Resistance vs Circuit Board Copper Area graph on datasheet  Pg. 23. the vertical axis indicates a junction-to-ambient thermal resistance θJA that is between 34 and 45 °C/W, depending on the copper area. Using this information and assuming a θJA of 35 °C/W recalculating TJ:

Tj= 85℃+(35 ℃/W*3 W) = 190℃

That would exceed the Absolute Maximum Junction Temperature TJ of +150°C. Therefore, the ambient temperature would have to be reduced or a more effective method of cooling would have to be employed to keep TJ of less than 150°C. If you follow the information provided in Figure 47, I think you should be able to determine the TJ for your specific conditions. Do note that the θJA listed in the thermal table is 33°C/W, which is close to the 35°C/W, that I used for my calculation.

I have submitted a request to our package thermal team to have the thermal parameters of the OPA564-Q1 DWP and DWD packages recharacterized. Once I have that information available the plan is to update the datasheet thermal table.

Regards, Thomas

Precision Amplifiers Applications Engineering

• Hi Thomas-san

Thank you for your detail explanation.

I understood that thermal parameters will be recharacterized.

BTW, please let me clarify θja.

Which(Top or Bottom) ambient does θja=33℃/W  mean ?

If θja mean at top side, θjc should be smaller than θja.

Best regards,

Shota Mago

• Mago-san,

The θja for the OPA564-Q1 DWP package would be with respect to the bottom side thermal path. θja (33℃/W) for the DWP package would be the sum of the thermal resistances in that path, θja = θJC + θCB + θpcb-A.

Where:

θJC is the thermal resistance from the OPA564-Q1 transistor junctions to the package PowerPad outer surface

θCB is the thermal resistance from package PowerPad outer surface, to the PC board copper pad to which the PowerPad is soldered

θpcb-A is the thermal resistance from the soldered PC board copper pad to the ambient air environment to which the heat is radiated

I do think that the OPA564-Q1 datasheet Fig. 47 provides useful information regarding the expected θja for a particular PCB copper heat sink area. The graph shows that for 2 oz. (56.7 grams) copper the area required for a θja=33℃/W, requires an area of about 4 sq-in (10 sq-cm), or larger.

Regards, Thomas

Precision Amplifiers Applications Engineering