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LM2903: There are batches of products with abnormal impedance to ground pins, please help check whether there is a problem with the design

Sam Sun
Intellectual 875 points
Part Number: LM2903


1. The file is a record of the customer's test. 2. The output frequency of the customer application is abnormal with the standard test frequency, the schematic diagram is as shown below, it is a channel of frequency input, PI1 is the frequency input of the product, and it enters the + input terminal of U56A of LM2903 through embedding and filtering; -The input terminal is to adjust the voltage of the threshold, PWM adjustment is carried out by the PWM_REF1 pin of the MCU, the output REF_PI1, the signal adjusts the output range of 0~5V, connected to the -input terminal of U56A, after the comparison of U56A, the output signal PIN1 is connected to the pin of the timing frequency of the MCU.

WIKA-LM2903-FA - Update2.docx

  • 0
    TI__Genius 11701 points

    Hi Sam,

    Please see section 2.2 - 2.4 of this app note: ti.com/lit/an/snoaa35c/snoaa35c.pdf?ts=1686246109088&ref_url=https%253A%252F%252Fwww.google.com%252F

    There is a current PCN for this family of devices so you may be getting some batches of the old die and some batches of the new die. The new die has lower input bias current which may be affecting your impedance measurements. 

    Can you elaborate on what you mean by the output frequency is abnormal? Do you have any waveforms to share between normal and abnormal?

    I would remove C2221 as this would add a delay to the hysteresis and effectively negate the hysteresis. 

    I would also either remove C220 or at least put a small output resistor between the output and C220 as you would not want to put a cap directly at the output of the comparator.