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LM1085: Temperature Response

Lucas Pucheta
Expert 4621 points
Part Number: LM1085

Vout= 5V 3A 

 Vin=12V

Consider the TAB being soldered to a 1”sq copper pad on the top side of the PCB. The copper pad has via's connecting top layer copper pad with bottom layer copper pad. 4-layer PCB.

Two questions:

  1. How long (transient pulse) could this part deliver at 1A, 2A or 3A without exceeding Tj at Ta=70C?
  2. What's the max continuous DC current this part without exceeding Tj at Ta=70C?

BR,

Lucas Pucheta

  • 0
    TI__Guru 50025 points

    Hi Lucas,

    We would need a thermal model to answer this question accurately. With that said, a quick way to estimate the max DC current is to use the thermal metrics from the datasheet. The board you have described is reasonably close to the JEDEC high-k board layout (which is what is used to get these thermal metrics) so they should provide a reasonable estimation. The max DC current for the NDE package would be (125C-70C) / (22.8C/W * (12V - 5V)) = 344mA, and for KTT package it is (125C-70C) / (40.6C/W * (12V - 5V)) = 194mA. Here you are limited by the headroom - 7V is quite a lot if you are hoping to drive a lot of current. I'm not sure exactly how the NDE package thermal metrics are obtained because the package is through-hole and I believe there is a heatsink attached to it, but I'm not sure how the heatsink characteristics are defined. Which package were you considering?

    If you're willing to wait for a thermal model I can put in a request for it. If so, we would need a little more information. 

    • Is the bottom layer pad the same size as the top layer pad (i.e. 1sqin)?
    • Are either of the internal layers connected by vias to the tab?
    • What are the board dimensions? I.e. board thickness and WxL

    Regards,

    Nick

  • 0 in reply to Nick Butts
    TI__Expert 4621 points

    This is an old 5V product that includes a host board and a 900MHz radio module.

    The radio vendor changed its power supply design such that when enabled, the immediate inrush current, ~1.5A, causes the hosts  5V 1.5A LDO input and output voltage to droop, causing the host board to reset.

     During normal operation the 12V idle current is ~100mA. When transmitting short 16ms current pulses of ~500mA occur. Duty cycle of these pulses is ~ 1%

     So we arelooking to tame this inrush current at the radio interface. Not sure if the 3A LDO will accomplish this. Perhaps an inrush current limiter?

  • 0 in reply to Lucas Pucheta
    TI__Guru 50025 points

    Hi Lucas,

    Are you looking for a P2P device for this then or is the board getting a rev? Do you have a schematic to share to help me understand the system? 

    If you can't do anything about the 1.5A inrush current, it might help to add more input and output capacitance to help limit the droop during the inrush current, although that will add more inrush current to the 5V LDO itself. What is the acceptable level of droop to avoid a system restart? I don't know what the power sequencing looks like so I don't know if this is feasible but I would think it's worth considering. 

    Otherwise, an external current limiting device could be helpful. What is the inrush current supplying? 

    Regards,

    Nick