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ULN2803A: Calculating maximum Power dissipation

Brian Farmer
Prodigy 20 points

Part Number: ULN2803A

For ULN2803ADW in Section 6.4 Thermal Information, RθJA Junction-to-ambient thermal resistance 66.4 °C/W is given. Other parameters are given as well.

I am confused because this Section references to Thermal Metrics, SPRA953C; which says, "Tj = Ta + RθJA*Power is often a misapplication of the RθJA thermal parameter because RθJA is a variable function of not just the package, but of many other system level characteristics such as the design and layout of the printed circuit board (PCB) on which the part is mounted.

Question, why make this complicated? Is your application based on JEDEC to give you 66.4 °C/W. If so, which one High L or Low?

How exactly did you arrive at 66.4?

In other words. Please show me step by step how to calculate worst case Tj at say 55C.

  • 0
    TI__Genius 10855 points

    Hi Brian, 

    Thanks for reaching out on E2E!

    The RθJA value shown in the datasheet is the thermal junction resistance measured on a JEDEC board. Since your solution / board might have a different design (different copper weight, thermal vias, layout) than the JEDEC standard, we can't quantify what your thermal resistance is. However, I can say that the JEDEC value is usually shown as a "worst-case" value, so your board should have better thermal resistance than the value shown in the datasheet. 

    Can you elaborate more on your system? If you provide more operating conditions, I can calculate your thermal information and see if you're violating any specifications such as the junction temperature. I also recommend taking a thermal camera and measuring the thermal dissipation on your system, this will provide the most accurate representation of your thermal resistance.

    Thanks,

    Arthur Huang