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Recommendation of a Linear Regulator for convert 12Vdc to 5Vdc with a maximum current of 150mA.

Oscar Gomez Fuente
Expert 2066 points
Other Parts Discussed in Thread: ISO3088, TPS7A49, TPS7A47, TIDA-00270

Hi,

I have to manage a linear regulator to convert 12Vdc to 5Vdc with a maximum current of 150mA. I had a look to the file ISO3088, and the TPS7A49 option It could be a good option but I'm not pretty sure if it's going to be too much current to be dissipated because I'm going to have a drop voltage of 12-5 = 7V.

I don't like to use a buck converter because I'm going to use the 5Vdc branch to feed DACs an ADCs.

It doesn't mind to use two linear regulators instead two to try to dissipate better the heat.

Two TPS7A17 It could be a good option too.

Could anyone give me a recommendation to this issue?

Best regards.

  • 0
    TI__Expert 3910 points

    Hola Oscar,

    There is a couple of things you have to take into account, such as your ambient temperature (TAmbient) and the maximum junction temperature (TJunction,max) as well as the thermal resistance which will vary depending on the package you choose to use. A good application note to review if you are not familiar with these terms is "Semiconductor and IC Package Thermal Metrics"

    We have a blog that talks about this "Watt’s wrong with my LDO?", the article describes the two simple steps you can take to select the right part:

    • Calculate the Power Dissipation (PD).

    PD = (VIN - VOUT) * IOUT

    • Calculate the minimum junction-to-ambient thermal resistance (RθJA) required, we know that TJunction,max = TAmbient + (RθJA * PD), so:

    RθJA ≤ (TJunction,max - TAmbient) / PD

    In your case, assuming a TAmbient = 25°C (make sure to take this into account), you need a RθJA ≤ (125°C - 25°C) / ((12 V - 5 V) * 0.150 A), or:

    RθJA ≤ 95.24°C/W

    For TPS7A49:
    RθJA(DGN Package) = 63.4°C/W
    RθJA(DRB Package) = 47.7°C/W

    I believe we do not have a TPS7A17 part, so I am assuming you meant TPS7A47 :
    RθJA(RGW Package) = 32.5°C/W

    For other LDOs, the RθJA will usually be available under “Thermal Information” and the TJunction,max is the upper end of the recommended “Operating Junction Temperature” usually found on the “Electrical Characteristics” table.

    If you are worried about your TAmbient being too high hen you could do current sharing as an alternative, in that case you could follow this TI Design: "Current-Sharing Dual LDOs" (TI Design Number: TIDA-00270)

    Best Regards,
    Victor

  • 0 in reply to Victor Salomon
    Expert 2066 points

    Hi Victor,

    Thank you very much for you lecture.

    Then assuming that Tamb = 25ºC with a PD = (12-5)*0.150A = 1.05Watts, I need a linear regulator with RθJA ≤ 95.24°C/W so TPS7A49 and TPS7A47, particularly the last one, are a good option, aren't they?

    Best regards.

  • 0 in reply to Oscar Gomez Fuente
    TI__Expert 3910 points
    Hi Oscar,

    Yes, both options are good, however keep in mind that your ambient might vary if you have any other elements that get hot near the LDO, if you ambient is higher TPS7A47 might be your best bet, you could reach ambient temperatures of 85°C and still be within the operating temperature range, on the other hand TPS7A49 would still fall within the operating range in between 55°C and 70°C, depending on the package you choose.

    Regards,
    Victor