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TPS7B7702-Q1: Thermal performance on EVM

Albert Sun
Intellectual 330 points
Part Number: TPS7B7702-Q1
Other Parts Discussed in Thread: TPS7B7702, TPS7B7702EVM

Hi Team,

I want to know about thermal performance for TPS7B7702-Q1. My input voltage is 13.5V and output is 8.5V. I want to understand what is the maximum DC current it could support.

My maximum ambient temperature is Tmax. Then the available power dissipation on LDO is (175-Tmax)/40.3. The power dissipation on single channel is 5V*IOUT=(175-Tmax)/40.3. So maximum output current is (175-Tmax)/201.5.

Temp(C) 25 45 65 85 105
IOUT.MAX(mA) 744.4 645.16 545.9 446.6 347.4

When temperature rise to 65C, TPS7B7702-Q1 could not support 300mA/channel when both channel is supplying same load. As Rja is based on JEDEC standard. The thermal performance will be different from EVM, which builds with heavier copper and stronger thermal dissipation. I want to know the Rja on EVM to evaluate how much maximum current TPS7B7702-Q1 could support. Could you please share some data? Thank you very much!

-A

  • 0
    TI__Mastermind 41495 points

    Hi Albert,

    Your analysis is sound. 

    You mentioned using both channels to supply the same load. What is your plan to use the device in that configuration? The outputs cannot be directly tied together. 

    You are correct, the EVM has a more thermally optimized layout than the JEDEC High-K board layout used for the simulations to produce the thermal characteristics. We do not measure the RθJA for the EVMs, and simulating it requires significant effort because all of the copper has to be manually placed for all of the traces and copper pours. 

    Do you have an EVM on hand? There is a pretty straightforward way of measuring the RθJA if you have access to an oven.

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Intellectual 330 points

    Hi Nick,

    I do not have EVM on hand and I wish to have this data quickly. I do not mean to use the rails in parallel because this will have current sharing issue without extra circuit designs. I am going to use one channel for a 800W camera and one rail for MIC. 

    The question is the camera+MIC's current consumption is a lot now. I want to evaluate how much DC current TPS7B7702 could go in 85C ambient.

    If you could help measure the highest current could go with TPS7B7702 EVM in your side then it would be greatly appreciated:)

    -A

  • 0 in reply to Albert Sun
    TI__Mastermind 41495 points

    Hi Albert,

    Got it. I will try going in and doing a measurement tomorrow or Monday. I'll update this post when I have info to share.

    Thanks,

    Nick

  • 0 in reply to Albert Sun
    TI__Mastermind 41495 points

    Hi Albert,

    I was able to drive at least 644mA (higher than the rated current in both channels) on the EVM at Ta = 85C, VIN = 13.5V, VOUT1 = VOUT2 = 8.5V. Once the current limit tripped for one channel the thermal dissipation in that channel goes up by 8.5V * ILOAD, so then the device starts cycling in and out of thermal shutdown, so I was not able to get the exact maximum, but this should serve as a good reference. See attached scope shots.

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Intellectual 330 points

    Hi Nick,

    Thank you very much! So 644mA per channel is the maximum DC current TPS7B7702EVM could support, right? 

    -A

  • +1 in reply to Albert Sun
    TI__Mastermind 41495 points

    Hi Albert,

    No, the current on the scope shots is the input current, i.e. the sum of the quiescent and load currents for both channels. Also keep in mind that, as was noted above, the EVM has a pretty well thermally optimized board layout and so you likely will not see that kind of maximum load current under these conditions unless your layout is also well optimized for thermal performance. 

    Regards,

    Nick