• State TI Thinks Resolved
  • Locked Locked
  • Replies 8 replies
  • Answers 2 answers
  • Subscribers 63 subscribers
  • Views 1862 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMZ35003: Maximum Operating Junction Temperature and Thermal Shutdown Question

KevinZhang123
Expert 5750 points
Part Number: LMZ35003

Hi, Team

My telecom customer is using LMZ35005 in their Smart Antenna product.

In LMZ35003 datasheet, the MAX Operating Junction Temperature is only 105°C, but the Thermal shutdown temperature is 180°C.

Why the thermal shutdown temperature is much higher than MAX operating junction temperature?

Will the LMZ35003 be failure if junction temperature is between 105°C to 180°C?

Thanks

Kevin

  • 0
    TI__Guru**** 191445 points
    The thermal shutdown is a failsafe feature and is always higher than the maximum junction temperature. The 105 C maximum operating junction temperature does seem a little unusual. The LMZ35003 is specific based on operating ambient temperatures. the Safe Operating Area curves in the data sheet imply that the actual maximum operating junction temperature is higher than 105 C. Lets see what the module support persons have to say about it.
  • 0
    TI__Guru 50120 points

    Hi Kevin,

    The absolute maximum rating of the LMZ35003 has an operating junction temperature of -40C to 105C as a module. Generally the thermal shutdown temperature is specific to the IC in the module, which after I checked, does have a higher operating junction temperature. Most likely, the passives bundled in the module( inductor, capacitor, etc.) are the components that are bottlenecking the LMZ35003's temperature rating. As a result, the datasheet for the module only guarantees max operation up to 105C before the device begins to experience damage.
     
    Regards,
    Jimmy

  • 0 in reply to Jimmy Hua
    TI__Expert 5750 points
    Hi, John and Jimmy

    Thanks for your explanation.
    LMZ35003 application spce: Vin = 10V~30V, Vout = 8V, Iout = 1A average, 1.5A max.
    Customer has tested their system in 30C ambient temperature. The top case temperature of LMZ35003 is 60C. So the temperature rise is 30C.
    Customer need ambient temperature up to 70C. So in 70C ambient temperature, the top case temperatue will go to 100C, and the junction temperature may bigger than 105C.

    Customer has mass production with LMZ35003 in other projects with Iout < 1A for several years. They can not change LMZ35003 to other devices in short time in this project.

    Is there any risk in this application? Any suggetion?

    Thanks
    Kevin
  • 0 in reply to KevinZhang123
    TI__Guru**** 191445 points
    Kevin,

    I do not support LMZ35003, but I have been discussing this with Jimmy directly. From what I can tell, the SOA curves in the data sheet indicate that LMZ35003 can support greater than 105 C junction temperature. Jimmy will verify if it is true or not. I just performed some calculations to satisfy my own curiosity and possibly help you out as well.
  • 0 in reply to KevinZhang123
    TI__Guru 50120 points

    Hi Kevin,

    Provided below are my calculation of the expected junction temperature the customer’s system will experience.

    Given Parameters:

    Vin= 10~30V, Vout=8V, Iout = 1A avg, 1.5A max, Ta=70C.

    Since the datasheet doesn’t have a graph for typical characteristics of Vin=30V and Vout=8V, I will pick the next highest one Vin=36V and Vout=5V at Iout= 1.5A. This selection would result in higher power dissipation than in the customer’s system.

    If we were to look at Figure 13 of the datasheet given: Vin = 36V, Vout=5V, Iout=1.5A, the efficiency is approximately 82%.

    The equation for power dissipation is Pdiss = Pout * ((1/eff)-1).

    The result is that Pdiss= 5V*1.5A*((1/0.82)-1)= 1.646W.

    The equation for junction temperature is TJ = TA + ( R θJA × PD ).

    Assuming that your board has a juntion to ambient thermal resistance of 14C/W following the datasheet,

    the result is that TJ = 70C + ( 14C/W × 1.646W ) = 93.044C.

    Since the datasheet specs the device to have an absolute maximum rating of 105C, the customer’s application should be fine. 

    Please let me know if you have any question on this calculation.

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    TI__Expert 5750 points

    Hi, Jimmy

    Customer use PCB with 1 OZ Cu weight, and no fan. I simulate in Webench, the junction temperature is 108C. I have shared the design to you via Webench.

    Any suggestion?

    Thanks

    Kevin

  • 0 in reply to KevinZhang123
    TI__Guru**** 191445 points
    When I run that simulation with 2.05 W dissipated I get 104 C junction temp. In any case, you are running close to the limit. I may also comment that the PCB used does not seem thermally optimized. I would move the bottom side components to the top and make the bottom layer GND continuous under the IC.
  • 0 in reply to KevinZhang123
    TI__Guru 50120 points

    Hi Kevin,

    The Webench thermal simulations look like it is using the size of the LMZ35003EVM which is 10cm x 5cm (4inch x 2in). The datasheet specs the ambient thermal resistance to be 14C/W when using a 10cm x 10cm board (Section 4.5)

    Most likely the actual thermal resistance is a bit higher than 14C/W that is in Webench simulation and would result in an increase in junction thermals.

    I wouldn't be too worried about the application where Iout<1A since you aren't pushing that much current through the device and the junction will not heat up that much. 

    Let me get back to you when I get more information from the Webench team on this particular simulation.

    Regards,

    Jimmy