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LM3940: Equivalent Series Resistance

Sean Maniar
Prodigy 10 points
Part Number: LM3940
Other Parts Discussed in Thread: TMP302, TLV755P

I have a few questions about a design I'm creating using the LM3940 voltage regulator to power the TMP302 temperature switch.

The first question is very straightforward. For the exact model of the LM3940, I'm deciding between the LM3940IMP-3.3/NOPBCT-ND and the LM3940IMPX-3.3/NOPBCT-ND models. I can't find any difference between the two, so if there isn't any I would just use the cheaper option.

The next question is similar to the other questions asked about the LM3940. I've attached the schematic for the circuit I'm making for reference. I see other people saying not being in the ESR will lead to instability, but all of those questions seemed like they were receiving a noisy supply power. I'm getting my voltage from the 5V line of a USB, which I expect to be far less noisy. If I use ceramic capacitors, will I run into the same problem with instability?

Lastly, I didn't use a copper pour on my PCB to dissipate heat from the regulator since I didn't want that to interfere with the operation of the temperature switch. Do you think the voltage regulator will still properly be thermally regulated without it?

Thank you in advance.

-Sean

  • 0
    Prodigy 10 points

    Schematic attached below.

  • +1 in reply to Sean Maniar
    TI__Mastermind 33745 points

    Hi Sean,

    The "x" version just describes the package quantity.  No X = 1000 pieces.  X = 2000 pieces.

    You'll still need to use capacitors that meet the ESR stability curve in the datasheet, regardless of what type of power source is feeding Vin of the regulator.  You can use a ceramic capacitor if you place a series resistor with it, like 300m-500m-ohms.

    It looks like the TMP302 draws 15uA so thermally you'll be fine.  The LM3940 has mA of quiescent current so you might see a temp rise from that. 

    A cheaper solution for you is probably the TLV755P.  The TLV755P is stable with ceramic capacitors so you don't need any extra resistors or components.  The quiescent current of the TLV755P is small, in the uA's.  Thermally you won't see a significant rise with 15uA of current, you'll have better noise and PSRR.  And the TLV755P is about 6 cents in the same package, however the LM3940 is like 10x more.  I would recommend the TLV755P for best performance and also the cheapest cost.

    Thanks,

    Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 10 points

    Hi Stephen,


    Thank you for such a thorough answer, you completely answered my question and I think I will use the TLV55P. Have a great day.

    Best,

    Sean