LM317A: Could you tell me about thermal design of LM317?

Part Number: LM317A

Hi team.

I have a question about LM317.

Could you tell me about it?

<Question>

In the datasheet p5 "7.4 Thermal Information" there is 23.3 (NDE: TO - 220 PKG) as "RθJA (Junction - to - ambient thermal resistance)".

What is the value of 23.3 specified in what mounting condition?

I think that this package called TO - 220 can be implemented as below.

· Attaching to the fin

· Direct mounting on board

By checking what kind of method is prescribed by your company,

We are considering whether customers can use it from the viewpoint of thermal characteristics.

Best regards,

Masumi Sekiguchi

8 Replies

  • Hi Masumi,

    If thermal is a concern, the best thermal performance will be achieved by connecting the TAB to a thermally conductive material in order to pull head out of the LDO. This could be either a heatsink or the Vout plane on the PCB.

    Very Respectfully,
    Ryan
  • In reply to Ryan Eslinger:

    Hi, Ryan-san.

    Thank you for contacting.

    I want to know is the following,

    Q:What is the condition of specifing ”RθJA= 23.3" ?

    Regards, Masumi.

  • In reply to Pokkun:

    Hi.

    How is the situation on this thread?

    Regards, Masumi.
  • In reply to Pokkun:

    Hi Sekiguchi-san,

    The measurement was taken with a thermal simulation with the device mounted on a standard JEDEC board.

    The standard JEDEC board used to generate the standard datasheet thermal table is 114.3 x 76.2 x 1.6 mm and has 4 layers.  The top and bottom layers are 2 oz Cu each and the buried layers are 1 oz each. There are thermal vias under the pad, the number of vias would be the maximum that would fit beneath the exposed pad based on 1.2 mm pitch.

    Regards,

    Mike

  • In reply to Michael Hartshorne:

    Hi, Mike-san.

    Thank you for your detailed answer.

    By the way,

    is this consideration OK that "the measurement conditions don' have heat radiation fins attached?"

    Best regards,

    Masumi Sekiguchi

  • In reply to Pokkun:

    Hi Sekiguchi-san,

    If the measurement were performed with a heat sink or fins attached, the RTja would be lower. The numbers given would be a conservative estimate for a standard PCB thermal dissipation condition - additional heat dissipating elements can only help.

    Does this answer your question?
    Regards,
    Mike
  • In reply to Michael Hartshorne:

    Thank you for your answer.

    The implementation condition when the definition condition of "23.3" this time is,
    Is it going with the heat sink pad mounted (soldered) to the PCB?

    Depending on the usage of the customer,
    the TO-220 may be mounted standing up in a 3 terminal point,
    there are cases where heat radiation pads are used without soldering mounting,
    my customer want knowing detailed mounting conditions.

    Best regards
    Masumi Sekiguchi
  • In reply to Pokkun:

    Sekiguchi-san,

    The RTja is specified with the exposed pad soldered on to the board.  If the device is mounted vertically, and a heat sink is used, the RTja can vary widely depending on the heat sink and connection to heat sink.  We don't offer RTja values for these conditions, the heat sink vendor should be able to offer a value of the expected RTja when the heatsink is used.

    Regards,

    Mike