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LM331: Power supply at 4.2V

john6869
Prodigy 60 points
Part Number: LM331
Other Parts Discussed in Thread: LM311

Hi team,

I have to make an electronics project based on LM331, specifically an F-to-V converter, but I can only use the 5V power supply from the USB output of a common PC.

As I know, the USB output from a PC changes from 4.5V to 5.2V approximately, so I was thinking in a LDO of 4.2V for the Vcc of the LM331, do you think it is a good choice?

Because I don´t want to use a Boost converter beause of the noise...

PS: the input frequency range goes from 100Hz to 10KHz and it´s a square wave (0V - 3.3V) so I also have doubts about the choice of components.

PS2: the project statement doesn´t talk about LM331, I choose this because as I´ve read has a good features but any other suggestions for my conditions are very welcome.

Thanks.

  • 0
    Guru 140200 points
    Hi John,

    as many of the important data are specified for a supply voltage of >=4.5V, I would try to take the USB voltage, but would introduce some proper supply voltage filtering for the LM331. It depends a bit on how noisy your USB voltage is. But I think it's a good idea to take a ferrite bead and a lossy electrolytic cap (aluminium electrolytic or tantal) paralleled by a 100...470nF X7R. The losses of electrolytic cap (ESR) help to prevent LC-resonances.

    Referring to the component choice, have you seen section 9.3.1 of datasheet? There are circuits shown which can be taken as a starting point for your experiments.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 60 points
    Is your solution only applicable to noise? I mean, the LM331 doesn't need a linear power supply to work?

    Sorry if it's a silly question
  • 0 in reply to john6869
    Guru 140200 points
    Hi John,

    the supply voltage of LM331 must be clean, stable and noisefree. But there's no need to use a linear voltage regulator.

    Kai
  • 0
    TI__Guru* 93105 points

    Hello John,

    I think the LM331 will do what you need, but it might take a more involved circuit than the simpler V-to-F converter circuits shown in datasheet Figures 14 and 16. I believe they provide a pulse output stream, and not a square wave output. Here is some information that may be helpful to you when applying the LM331 (courtesy of Mr. Paul Grohe, of our Santa Clara team):

    • Most common mistake is making “RS” too low and overloading the current source. Should be around 14k

    • No lower frequency limit – limited by leakage (and soakage) of large caps.

    • Upper limit around 1MHz, but accuracy suffers greatly. The frequency graph
    is there for a reason!
    • Helpful Appnotes:
    –AN-C, “V/F Converter ICs Handle Frequency-to-Voltage Needs” (SNOA734)
    See Figure 15. Sine-Wave VFC to Use with PLL. The circuit diagram shows a 0/15 V square-wave output after the the LM311 comparator and Q3 output.
    –AN-D – “Versatile Monolithic V/Fs can Compute as Well as Convert With High Accuracy“ (SNOA735)
    Use this link to get to the Applications Report: http://www.ti.com/lit/an/snoa735b/snoa735b.pdf

    Is Kai's USB power supply advice helpful to you? If so, and you have everything else you need we would appreciate it if you could close this e2e thread.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 60 points

    Hi Thomas,

    I really appreciate your time spent in that complete reply, but I was asking for a F-to-V converter.

    Also I will take some advices from the notes that you share.

    Thanks

  • 0 in reply to john6869
    TI__Guru* 93105 points
    Hi John,


    Thanks for the clarification about using the LM331 for the F-to-V conversion. I somehow turned things around in my mind and was thinking V-to-F. The LM331 datasheet, and especially application note SNOA734B, provide much useful information about applying the LM331 as an F-to-V converter.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    Prodigy 60 points

    Hi Thomas,

    That application note is really helpful, thank you so much.

    John