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heating LP38856S-1.2

We have designed LM1085IS-3.3 LM1086IS-2.5 LP38856S-1.2 in our Product.

LP38856S-1.2(DDPAK/TO-263 (KTT) package) is heating up above 90 degree. The other devices is no problem.  

The schematic is below :

Please let me have advice.

  • "... above 90 degree ..."

    I presume this is 90°C. Yes?

    Package dissipation (Watts) is a function of Vin, Vout, and Iout.

         Pd = ( (Vin - Vout) x Iout )

    The internal junction temperature (Tj) is related the above Pd value (W) along the with measured external temperature (Tcase, or Tboard) and the respective thermal gradient values (PsiJC, or PsiJB).

    For LP3885x family the dissipation from the BIAS input is usually very a small value (5V x 4.5mA = 22.5 mW) that can be ignored.

    Without knowing the load current (Iout), and where the temperature is measured, it's difficult to guess.

    The bottom line is that this temperature might be expected operating behavior.

    If Vin, Vout, Iout, and the package type are set, the only remaining variable would be to lower the overall thermal resistance: Theta(ja). This is simply improving the transfer of heat from the package to the ambient air. This could be through the PCB by adding more copper area (and/or thicker copper) under the device thermal tab, adding thermal vias from the thermal tab to an inner ground plane. Or, it could be adding a heat-sink to the top of the package case.

    Again, knowing the load current (Iout), and where on the package the 90°C temperature is measured, would be helpful.