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LM331: Reference design for 5 volts power supply

user4303883
Intellectual 390 points
Part Number: LM331
Other Parts Discussed in Thread: TPS7A4001

Dear,

I need to measure DC voltage between 5 volts up to 110 volts with voltage to frequency converter, my idea was to use a voltage divider first, and LM331 followed by an optocoupler in order to connect to MCU. I have no power supply available except the voltage to be measured , so in order to power LM331, I can use step down (I found no at TI max = 100 Volts, but found LTC 3638 at linear).

I have no problem of fast sample , only one per second is enough.

so my question are :

1) does the voltage divider followed by LM331 and optocoupler seems to be acceptable design ?

2) I did not find any reference design with 5 Volts power supply for LM331, is this somewhere available ?

Best Regards

  • 0
    Guru 140200 points

    Hi,

    where is the 110V coming from? Mains voltage?

    Kai

  • 0 in reply to kai klaas69
    Intellectual 390 points

    Hi

    110 Volts comes from a battery , but it can be 24 or 48 or ..

    Rgds

  • 0 in reply to user4303883
    Guru 140200 points

    Hi,

    running the LM331 at 5V supply voltage is possible, but keep an eye on the limited common mode input voltage range of -0.2V...Vcc-2V, according to the datasheet.

    Using a resistive voltage divider should also work, if you divide the 110V down to 3V. You could do it this way:

    Kai

  • 0 in reply to kai klaas69
    Intellectual 390 points

    Thanks can we estimate easily the precision of measurement with the LM331 in this configuration ?

  • 0
    TI__Guru* 93105 points

    Hello user4303883,

    The LM331 is usable with a supply voltage range from +4 V to +40 V. If you can maintain at least +4 V on the Vs pin then the LM331 should function well as an accurate V-to-F converter. Although I don't have a specific V-to-F +5 V applications circuit, there is a TI Applications report for the LM331 which should be of help in applying the device:

    http://www.ti.com/lit/an/snoa735b/snoa735b.pdf

    The LM331 draws about 3 mA of supply current from a +5 V supply so you need to keep this in mind should you decide to try and apply a resistive divider to power it. Alternately you may be able to employ an LDO to power the LM331 from the HV input.

    Low dropout voltage regulators (LDO) don't usually go up to a high voltage such as 110 V. However, TI has a low power LDO, the TPS7A4001, which allows for an input voltage up to 100 V. You could set up a 90 % resistive divider to drop the 110 V to 100 V, and have the TPS7A4001 output set for a 4 V output. Then when the input voltage is at the minimum 5 V level, the LDO input would be +4.5 V; enough for the LDO to maintain the +4 V output to the LM331 Vs pin.

    Here is where you can find more information about the TPS7A4001 LDO:

    The LM331 datasheet outlines the performance you can expect from the LM331 connected as a V-to-F converter. 

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Intellectual 390 points

    Hello Thomas,

    Thanks for the info, I will look at document.

    regards