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TPS7A25: thermal issue

Sky Tsai
Prodigy 10 points
Part Number: TPS7A25

Hi Sir/Madam,

i check the TPS7A25 datasheet.

i have a source 12Vdc. output around 3.3 V / 1A.

1.

T A(MAX) = T J(MAX) – (R θJA × P D )

T A(MAX) =125°C – [73.3°C/W × (12 V – 3.3 V) × 0.09 A] = 70.2°C

does it mean the contact max thermal of TPS7A2533 is 70 °C ???

2.

i measure the contact thermal of TPS2533 in my system. it's around 105°C. 

T J = R θJA × P D + T A

      = 73.3°C/W × (12 V – 3.3 V) × 0.09 A  +105

      = 162 °C. 

could you suggest how to solve this problem?  or change another LDO (pin to pin)

thanks.

  • 0
    Prodigy 10 points

    mistype , sorry.

    correct " output around 3.3 V / 0.1A"

    thanks.

  • 0
    TI__Mastermind 37965 points

    Hi Sky,

    1. I calculated the max ambient temperature differently than you. I put in the same numbers and got 67.6°C. This is to say that if the device is dissipating (12-3.3) * 90mA = .783W, the ambient temperature should be less than 67.6°C for its junction temperature to remain under 125°C. This is also assuming that your board layout is identical to the JEDEC High-K board. 

    2. What do you mean by "contact thermal"? Did you measure the top-of-case temperature with a thermocouple? In general you should not use the RθJA for on-board junction temperature measurements. RθJA is really only useful during the design process. If you measured the top-of-case temperature, you should use ψJT to estimate the junction temperature. 

    Please share the ambient operating temperature for your system and if possible the board layout of the LDO. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 10 points

    Hi Nick,

     it already solve this problem after use  ψJT to calc the junction temp.

    Thanks for the information.