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TPS61085 Switcher Pro output current and temperature

David Escandon
Expert 2175 points

Other Parts Discussed in Thread: TPS61085, TPS61087

I discovered that if you use Switcher Pro to design a boost application using the TPS61085, the software does not correctly specify the limits of the chip. This chip cannot provide more than1A of output current. Anything above this will force the chip into thermal shutdown.

You will need to refer to the datasheet for exact calculation, but a more realistic value (taking into account power dissipation and increase in junction temperature), is between 800mA and 900mA (max). For higher output currents, the better device to use the TPS61087, which has a thermal pad connection to dissipate more heat.

 

-David

  • 0
    TI__Guru**** 152430 points

    With a 2A switch current limit, the TPS61085 can provide more than 1A of output current in some applications.  Page 9 of the datasheet shows this.  The 3.2A switch of the TPS61087 allows it to provide more current.

    Regarding thermals, yes a device with a properly used exposed thermal pad allows a lower thermal resistance from the IC to the PCB.  This allows heat to leave the IC while creating less of a temp rise.  The thermal limits of the IC in a particular application are much harder to estimate as they depend on many system parameters: PCB thickness, PCB layout, PCB copper thickness, system airflow, ambient temperature, etc.

    Also, note that significant power is dissipated in the external components of the diode and inductor.  This is not heat in the IC and does not directly contribute to the IC's temperature.

    I will move this post to the switcher pro forum for them to comment on how the software calculates the temperature of the IC and its maximum output current.

     

  • 0 in reply to Chris Glaser
    TI__Prodigy 140 points

    Hi David,

    Thank you for your forum input. We are investigating your observations and will inform you of the outcome.

    Regards,
    Sameer

  • 0 in reply to Sameer Mylavarapu
    TI__Prodigy 140 points

    Hi David,


    • Following is a basic check on the inputs for TPS61085 in SwitcherPro.

    iSwitch = 1.1 * IoutMax * (Vout / VinMin)
    If iSwitch is greater than 2A, SwitcherPro does not proceed with creating a design.

    • Also, in the Stress tab of the SwitcherPro, the Rated Current(RMS) for the controller is incorrectly shown as 5A. We will schedule a task to fix this.

    • This is how SwitcherPro calculates the temperature of the IC.

    Temp. (C) = TotalLoss * Θja + 25

    where:
    TotalLoss = Loss(high FET + low FET + controller)
    Θja is Junction-to-ambient thermal resistance


    Please let us know if you have any questions.

    Thanks,
    Sameer