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Original question:

OPA564-Q1: θja vs. θjc

OPA564-Q1: θjc and θja Question

Lanxi Li
Expert 6085 points
Part Number: OPA564-Q1

Hello team,

About the resistance of θjc and θja, I have read these two reply in the E2E, but I still confused about the θjc and θja

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/786570/opa564-q1-ja-vs-jc?tisearch=e2e-sitesearch&keymatch=OPA564-Q1#

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/649014/opa564-thermal-issue

From the reply, you explain the difference of θjc top and θjc bottom and θjc bottom is the main thermal path, which can calculate like this θja = θJC(bottom) + θCB + θpcb-A. But from other thermal path, θja = θJC(top)+ θCA, how can I understand that θjc top (50 C/w) is still larger than θja (33 C/w) as the picture showed ? When OPA564-Q1 works, the temp of top case is larger than junction? This is really confuse me, Please help with this.

Thank you so much.

Best regards,

Lanxi Li

  • 0
    TI__Guru* 93105 points

    Hello Lanxi,

    I think the thermal models have made all of this difficult to understand and understand how to most effectively remove the heat from an IC package. The OPA564-Q1 has two methods by which heat are transferred from the semiconductor die to the outside world. The first and primary is by conduction from the die through the thermal pad to the PC board and then some through the package leads to the PC board, and the other via radiation from the package body and leads.

    The vast majority of the heat generated by the OPA564-Q1 die will be through conduction which provides the lowest thermal resistance path. Heat flow, like current flow, takes the path of least resistance. Therefore, what of primary importance in the thermal model you show is θjc bottom which has the low value of 1.83°C/W, plus whatever your PC board θpcb-A . Do your Tj calculation based on that thermal path and the average power dissipation. That should get you very close. 

    A small amount of the heat will be conducted from the die through the package plastic and then radiated by the package surface and leads. But because the thermal resistance is the much higher θja, its contribution to Tj temperature will be will be small. If the OPA564-Q1 package didn't have a thermal pad and direct contact to the PC board other than the leads then the package radiation would play a lot larger role in dissipating the heat.

    Here's are two TI Application's reports about package thermal metrics:

    https://www.ti.com/lit/an/spra953c/spra953c.pdf

    https://www.ti.com/lit/an/slua844b/slua844b.pdf

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Expert 6085 points

    Hello Thomas,

    I want to make sure that whether I get your meaning correctly.

    Do you mean that the θjc(top) = 50 is the value which is not using thermal pad, only with the package radiation? If there is no thermal pad, only package radiation, the θjc(top) = 50, the corresponding θjc is not 33?

    Also, θjc=33 we showed in the datasheet is the value we use thermal pad as main conduction path?

    If I am right, if my customer use the thermal pad to keep a primary thermal path, what is the right θjc(top) value when they in this situation?

    (Last, I want to clarify why mu customer need the thermal value, they will add sensor on the top of the package to detect the package temp and make sure the junction temp is in the safe state, so they need the right value to do the calculation)

    Thank you so much.

    Best regards,

    Lanxi Li

  • 0 in reply to Lanxi Li
    TI__Guru* 93105 points

    Hello Lanxi,

    The points I have made are with regard to the OPA564-Q1 DWP PACKAGE, PowerPAD on Bottom. 

    Regarding your questions:

    Do you mean that the θjc(top) = 50 is the value which is not using thermal pad, only with the package radiation? If there is no thermal pad, only package radiation, the θjc(top) = 50, the corresponding θjc is not 33?

    The OPA564-Q1 thermal numbers are derived by thermal modeling. It is my understanding that the package is modeled alone in 3D space so that just the thermal characteristics of the package are derived. The the θjc(top) = 50°C/W is the thermal resistance specifically determined for the center of the package top surface. I believe the θjc = 33°C/W is the package integrated thermal resistance. Since the OPA564-Q1 is intended to have the bottom thermal pad soldered to the PC board the very low θjc bottom of 1.83°C/W is the primary mechanism of package heat removal.

    If I am right, if my customer use the thermal pad to keep a primary thermal path, what is the right θjc(top) value when they in this situation?

    Since the thermal pad is on the bottom the θjc bottom that is the number of primary concern. If the customer is instead using the DWD package then the other numbers in the datasheet Electrical Characteristics table apply. The notes below the table provide some additional information about the thermal modeling for that package.

    The customer certainly can attach a thermal sensor to the top of the OPA564-Q1 package, but be aware thermal modeling is "ideal" and actual measurement will almost certainly provide a different result. I like the actual measurement because no assumptions enter into the results obtained.

    Regards, Thomas

    Precision Amplifiers Applications Engineering