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WEBENCH® Tools/TPS799: safe operating area for the adj version versus the fixed

Part Number: TPS799

Tool/software: WEBENCH® Design Tools

I am using webench to run some simulations on the part itself and came across a difference on a LDO when using the fixed output voltage versus the adjustable.  
The part in question is TPS79901 for adj and TPS79918 for fixed.  
Vin 5v
Vout 1.8v
Iout 0.1A 
If I switch Iout to 0.2A, the TPS79918 is no longer a viable part, but the TPS79901 is.  
I was looking at the safe operating area for the TPS79918 when Iout is set to 0.1A.  It seems I cannot go above Iout 0.13A  (Vin 5v) with the fixed version, but the adjustable I can go all the way to 0.2A.  
I would like to understand why?  
  • Hi Richard,

    The safe operating area for TPS799 will depend on which package you use as each package yields different thermal metrics.

    You can estimate the SOA by using the power dissipated (Pd = (Vin - Vout) x Iout) and the data provided in the Thermal Information table.  Since layout affects the thermal performance of the LDO, it is common to initially use the thetaJA information for early calculations. Tj max for TPS799 is 125C per the Recommended Operating Conditions Table.  Therefore we can graph the SOA using the following equation:

    Tj = Ta + thetaJA x Pd

    You are correct that there should not be a difference between the SOA for the fixed device verses the adjustable device given the same ambient temperature, package, and power dissipation.

    Very Respectfully,


  • Thanks Ryan for clearing this up for me and providing more detail to clarify how the Tj is determined. I just wanted to make sure I was not missing anything in regards to the adj versus the fixed.