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Thermal shutdown on LDO's

Randy Mitchell
Intellectual 330 points
Other Parts Discussed in Thread: TPS62110, TPS7A80

Needing a 5 volt voltage regulator with 500ma output and a input voltage of 12 volts I selected the Reg103-5 SO-8 package, but then found that I neglected the Theta Ja and the PD = (Vin -Vout) * Iout was  causing thermal shutdown due to Tj = Ta + (Vin - Vout) *( Iout*Theta Ja).

There was no way I was going to get to 500ma with the Reg103-5 using the SO-8 package with a 12 volt input requirement.  The DDPAK and SOT-223 packages provided better performance but require a lot more board space of up to 4 square inches of heat sink copper.

After futher review, it appears that the TL750M05 maybe a better option.  Unless the spec sheet is lacking / missing information, the TL750M05 does not appear to require as much board space for the heat sink and it does not mention the (Vin - Vout) parameter as being a factor in the Power Dis of the device.

Will indeed the TL750M05 DDPAK / TO-263 package device deliver 750ma of current with the suggested PC layout mounting / heat sink shown in the spec sheet?

Thank you for your help in advance!

Randy Mitchell

Design Engineer

Spectral Dynamics

  • 0
    TI__Guru*** 146540 points

    Randy,

    Any linear regulator will dissipate (Vin - Vout) * Iout in the pass element.  This is inherent in the linear regulator topology.  For LDOs, the pass element is inside the IC.  For linear regulator controllers, the pass element is outside the IC.  You will need a huge package and lots of copper area to successfully dissipate that much power, 3.5W, in a linear regulator design.

    Here is a thermal calculator tool that should be very useful: http://focus.ti.com/adc/docs/midlevel.tsp?contentId=76735

    And here is an app note discussing thermal concerns: http://focus.ti.com/lit/an/slua566/slua566.pdf

    An alternative to a linear regulator is a step-down or buck converter.  We have many of these to choose from and most operate at efficiencies above 90%.  The thermal challenges are far less with buck converters, but they do require much more electrical engineering knowledge to implement the LDOs do.  You can find one that meets your requirements at power.ti.com.  The TPS62110 would be a good fit.

  • 0 in reply to Chris Glaser
    TI__Expert 8890 points

    Randy,

    The only other suggestion I might offer is to use the TO-220 package.  Many customers have applications where they must use a linear regulator despite large power dissipation and so they use the TO-220 (KCS)  package screwed to a heat sink. 

    Regards

    Bill

  • 0 in reply to Chris Glaser
    TI__Mastermind 44630 points

    Hi Randy,

     

    If you need a clean power supply for your system, then using the TPS62110 to step down from 12 to maybe 5.5V is more efficient. Then you can use a TPS7A80 to post regulate a very low noise rail to your 5V load.

     

    http://focus.ti.com/docs/prod/folders/print/tps7a8001.html

     

    Regards,

     

     

    John

     

  • 0 in reply to JCHK
    Intellectual 330 points

    Thank you for your reply.  I now have a better understanding of the problem.

    Randy