LM331: Reference design for 5 volts power supply

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Replies: 6

Views: 71

Part Number: LM331

Dear ,

I did LM331 board as expected, but I have a strange signal output which is coming worst with voltage increasing :

any idea ?

Best regards

6 Replies

  • Please click on Insert/Edit Media button above and attach the schematic of your board showing all external component values, voltages and input signals. 

    Please note that Frequency Output, pin 3, is an open-collector output and thus requires a pullup resistor to work properly.

    Marek Lis, MGTS
    Sr Application Engineer
    Precision Analog - TI Tucson

  • In reply to Marek Lis:

    please find here my schematic :

    Thanks

  • In reply to Marek Lis:

    Dear Marek,

    Any advise about this request ? could you look at my schematic please.

    Thanks

    regards

  • In reply to user4303883:

    The LM331 frequency output, pin 3, is an open-collector output that requires a pullup resistor to work properly but your circuit has pin 3 floating - see below in blue box.

    In order for the circuit to work properly, you must connect pin 3 thru 10k resistor to Vlogic - see in red box below.

    Marek Lis, MGTS
    Sr Application Engineer
    Precision Analog - TI Tucson

  • In reply to Marek Lis:

    Dear Marek,

    I am sorry but I did not show you the right part of schematic, but I already have this pullup resistor, see below, so the problem is elsewhere.

    Regards

  • In reply to user4303883:

    Hi user4303883,

    Let's assume that you don't have a circuit connection problem on your LM331 PC board, then something else is preventing the circuit from functioning as expected. Here are some things I would check:

    1. Ideally, the Gain Adjust resistance RS connected to the LM331 pin 2 is 14 kOhms. If RS in FIgure 14 is set too low the current source will be overloaded and the LM331 will not function properly.
    2. The LM331 input comparator pin 7 input has an input current of about -80 to -300 nA. The voltage input of the V-to-F converter is intended to be driven by a low impedance source; however, in the case of your circuit the input filter R3, C3 looks into a high impedance (~R1 || R2). Try removing R1 and driving the input to the filter directly with a low voltage dc source. See if the V-to-F begins to function correctly. If it does, then the HV divider will need to be buffered before the LM331 input. A low power CMOS rail-to-rail input/output op amp would satisfy that purpose.
    3. CL in the LM331 datasheet is specified as 1 uF, while in your circuit it is 100 nF. The 47-Ω resistor in series with the 1-μF capacitor (CL) provides hysteresis which helps the input comparator provide high linearity. I suspect the V-to-F will function with the different capacitor value, but it may be worthwhile to change it to 1 uF if nothing else gets the circuit to operate.
    4. At some point you may have to try replacing the LM331. It may have been damaged during handling, or installation on the PC board.

    Do let us know if any of this helps get the circuit functioning.

    Regards, Thomas

    Precision Amplifiers Applications Engineering