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Questions on LM1084 LDO Regulator power dissipation

Dan Kamat
Prodigy 20 points
Other Parts Discussed in Thread: LM1084, LM1085

Hi -

Hopefully simple questions!

In Figure 15 of the datasheet for the LM1084 LDO regulator (http://www.ti.com/lit/ds/symlink/lm1084.pdf), I assume that the Y-axis (Power) is the amount that can be dissipated before the regulator is at risk of damage.

Assuming the regulator is damaged by dissipating too much power at high temp, what are the most likely manifestations of a failure?

     No voltage transferred to Vout?

     No voltage regulation on Vout?

     Would the failure be catastrophic / permanent?

     Or is the part potentially protected by current limiting circuitry and simply requires Overload Recovery (power recycle on the regulator)?

     Or is the failure non-catastrophic and simply decreases the expected useful life / reliability?

Thanks!

Dan

  • 0
    TI__Mastermind 22610 points

    " ... that can be dissipated before the regulator is at risk of damage..."

    No. This is the amount of power that can be dissipated for the TO-220 package (T), for a range of case temperatures, and the internal juction temperature still be within the ensured operating range (-40°C < Tj < 125°C).

    As Tj rises above about 150°C, the Thermal Limiting circuit will become active. This will reduce the available output current (which will cause the output voltage to fall) until the device has cooled, at which point normal operation resumes, no power cycle is required.

    The datasheet states : "...The LM1084 has different temperature specifications for two different sections of the IC: the control section and the output section ...". This statement is only valid for the first few second of a thermal transition. Within a short period of time the entire die will reach the same temperature. In this case the Max operating junction temperature of the Control Section (125°C) becomes the limiting temperature. (The LM1085 datasheet has the THERMAL CONSIDERATIONS section rewritten to presume continuous operation).

     

  • 0 in reply to Donald Jones
    Prodigy 20 points

    Thanks for the quick reply.


    To test my understanding, if I suspect that I'm pushing the input junction temp past 125C or the output junction temp past 150C, I could put a thermocouple on the case and monitor Vin and Vout to see a droop on Vout when the over-temp induces higher current draw which is then subjected to limiting by the internal protection.

    So no significant chance of catastrophic failure.  At least that's good news.

    Thanks again,

    Dan

  • 0 in reply to Dan Kamat
    TI__Mastermind 22610 points

    " ... no significant chance of catastrophic failure ..."

    That is correct.