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LMZ23608: Max load thermal derating

Sutej Torvi
Prodigy 200 points
Part Number: LMZ23608

Hi.

We are using this IC for our application.

And our use case is 24 Vin to 3.3V Vout @ 8A load.

As per the datasheet's thermal derating curve, the max load current for thetaJa 9.9 degC/W is around 6A for the ambient temperature 80 degC (which is our use case).

And for thetaJa 6.8 degC/W and 5.2 degC/W with TA = 80 degC, the IC supports for max load(8A).

So how do we ensure that the thetaJa is below 6.8 degC/W with TA being 80 degC?

Suppose we use the IC with thetaJa obtained at 9.9 degC/W, how do we make sure that at TA 80 degC, the IC is capable of supplying max load (8A) .

Do we need to use the heat sink?

Thanks, 

Sutej Torvi.

  • 0
    TI__Guru 50120 points

    Hi Sutej,

    Let me go over the process to calculated the PCB area needed for your application without airflow or heatsink below:

    The datasheet provides a handy power dissipation and thermal consideration calculation in Section 10.3.

    For your application of 24Vin, 3.3Vout, 8A load, at 80deg ambient,  the P_ic_loss is measured to be roughly 6W (Figure 18 black trace). Plugging this back into the equation, theta_CA is calculated to be 6.5W/C. 

    A good estimated of the required board area covered by 2-oz copper on both top and bottom metal layers for your application comes out to be 76.9cm^2.

    If the calculated board area above is too big for your application then an external heat sink may be required to help dissipate the heat. In any case for PCB design please follow the layout guidelines in Section 10.1, especially the heat-sinking vias to help wick the heat out of the device. 

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    Prodigy 200 points

    Hi Jimmy.

    Thanks a lot for your reply.

    The board area would be quite big for us!!

    However we will ask with our customers for the heat sink compatibility.

    Will definitely follow the layout guidelines mentioned.

    Thanks and Regards,

    Sutej Torvi.