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TPS7A45: Effect of junction temperature on current limit/ Clarifying TPS7A45XX Datasheet Figures 20&21

James Strickland1
Prodigy 20 points
Part Number: TPS7A45

Hello there,

I found a previous thread somewhat helpful as it clarified that the TPS7A45XX has current limiting protection which limits the output current depending on Input-Output differential voltage and ambient free air temperature, and is not based solely on junction temperature. 

Previous thread: https://e2e.ti.com/support/power-management/f/196/p/698922/2582061#2582061?jktype=e2e

I am left with two questions:

1. What is meant by "T_A - free air temperature" in figures 20 and 21 of the TPS7A45XX datasheet (included below)? As the package heats up during operation, am I to assume that the current-limiting circuitry is responding to a bulk air reservoir temperature and is isolated from the heating of the package? Is there a more precise definition that can help me understand the relationship between the current limit and the junction and/or package temperature?

2. Several responses to 'thermal protection' and 'current limit' questions infer or state that the current limiting circuitry, while not primarily dependent upon, is still affected by how the part is heat sunk. In my application, the TPS7A4501KTTR (DDPAK) is mounted such that it has a conductive thermal path with resistance of <5°C/W from its junction to an external reservoir. In this instance, based on the responses in the thread linked above, I expect the current will not be limited by junction temperature or the amount of power I can dissipate in the package. I would like to understand what I can expect the output current to be limited at given a much lower junction-ambient thermal resistance than figures 20 & 21 are supposedly generated from.

Any help you can offer will be greatly appreciated. I plan on conducting a test regardless, but I would like to understand this better beyond any empirical measurement.

  • 0
    TI__Mastermind 43630 points

    Hi James,

    Let me try to answer your questions:

    1. What is meant by "T_A - free air temperature" in figures 20 and 21 of the TPS7A45XX datasheet (included below)? As the package heats up during operation, am I to assume that the current-limiting circuitry is responding to a bulk air reservoir temperature and is isolated from the heating of the package? Is there a more precise definition that can help me understand the relationship between the current limit and the junction and/or package temperature? * I always interpret T_A or TA as the PCB ambient or the actual temperature of the device in the application. The current and thermal limits are based on measurements on the die alone. The best way to look at this is with respect to RTJA which is 28C/W in the TO-263 package. With this, during normal operation, you can predict the die temperature via TJ=TA+(VIN-VOUT)*ILOAD. 

    2. Several responses to 'thermal protection' and 'current limit' questions infer or state that the current limiting circuitry, while not primarily dependent upon, is still affected by how the part is heat sunk. In my application, the TPS7A4501KTTR (DDPAK) is mounted such that it has a conductive thermal path with resistance of <5°C/W from its junction to an external reservoir. In this instance, based on the responses in the thread linked above, I expect the current will not be limited by junction temperature or the amount of power I can dissipate in the package. I would like to understand what I can expect the output current to be limited at given a much lower junction-ambient thermal resistance than figures 20 & 21 are supposedly generated from. *The Current limit and Thermal shutdown for any LDO is present to protect the device. Normally what happens with an LDO during an over-current event is that first, the current limit kicks in which results in VOUT falling. As the differential between VIN and VOUT increases, the power dissipation increases.  When the die temperature TJ exceeds the thermal shutdown threshold, the device will shutdown thermally, Once the temperature cools sufficiently it will attempt to restart. 

     

    You are correct in that the RTJB is 5.1C/W. This is useful for modelling the effectiveness of our copper pour. The best tool to predict TJ is with RTJA as described above. 

     

    Does this help to answer your questions?

  • 0 in reply to JCHK
    Prodigy 20 points

    Thank you for your response, JCHK.

    Your description of how to interpret T_A was very helpful and I believe it answers both my original questions.

    Your final statement made me think that I may not understand the thermal information on the data sheet. I would like to clarify my understanding.

    The thermal information on the TPS7A45XX datasheet references TI's Semiconductor and IC Package Thermal Metrics application report, SPRA953. In it, it thoroughly describes how each of the thermal metrics is measured. Both junction-to-ambient and junction-to-board measure thermal resistance values when the device is mounted on prescribed test boards/coupons, and have very specific setups that are unlikely to be similar to practical applications. Junction-to-ambient uses a test coupon that must rely on radiation and convection to dissipate heat into still air (air reservoir is 'ambient'). Junction-to-board uses a test board which is cooled  with a cold plate in physical contact with the test board at prescribed distances from the mounting of the device. Please forgive me if the details aren't exactly correct, but my point is, each measurement fails to isolate the device so one can insert it into another model with different mounting and heat paths.

    For my modeling, I used the junction-to-case (bottom) thermal resistance of the TO-263 package of 0.3°C. (The 5.1°C/W you referenced is the junction-to-board of the SOT-223 package). I then have a model of the PWB on which the part is mounted with thermal vias and a thermally conductive heat sink which makes physical contact with the bottom of the PWB underneath the mounted TPS7A45XX. It is this thermal path for which I have an upper bound of 5°C/W thermal resistance from the TPS7A4501KTTR's  junction to my applications true ambient heat reservoir. 

    With the assumption that T_A is the package temperature, I can conservatively take package temperature<case(bottom) temperature and use that temperature together with Figures 20 and 21 on the datasheet to get a sense of how the current is going to be internally limited. 

    If I have this correct, then JCHK's first response answered my question and I consider this resolved. Otherwise, please correct my understanding.

    Thank you again, JCHK. I appreciate you taking the time to give me a thoughtful and well-written response.