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LM331: LM331 Voltage to Frequency conversion problems

Part Number: LM331

I am trying to debug 50 boards that are implementing the LM331 as a voltage to frequency converter. My desired frequency is 3.9 kHz. On 10 of the boards the frequency is within +/- 2kHz which is okay for my application. However on the rest of the boards the frequency is more like from 7.2 kHz to 8.4kHz. And i havent founf any difference regarding to the voltages on the pins.. The voltages at all the other pins are the same. All my resistors  are 1% and the caps are 5%. I've tried changing Rt, Ct and the LM331 but the result is still the same. My schematic is shown with a 3.6V input. Any suggestions as to why this is? Disregard the notes on the schematic. Also any other solution other than that of putting POT. Thank You

  • Hi Shoaib,

    If all voltages look correct, I'd suspect a leakage current, for instance at the reference current node. Since the LM331 will attempt to hold that output at ~1.9V, regardless of load, changes in R19 would change the reference current, and therefore the frequency. If possible, clean any flux residue from the board around the circuit.