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SN74HC165: SN74HC165 Input Short to Ground Issue

Part Number: SN74HC165


We are using SN74HC165 to read the parallel inputs from field& convert to serial. Attached the  circuit for one of the input, such a way all 7-input are connected to SN74HC165.

Based on the analysis of field failures, we observed that many of the inputs are shorted to Ground. Though the voltage& current to the input pins are limited by resistors, what could be the cause of the failure?

  • Hi Prabu,

    My first recommendation would be to use a regulated 5V supply instead of a voltage divider from 24V. It would be more power efficient and less likely to cause problems.

    Also, it looks like the input is being subjected to very slow edges, which could cause excessive current and damage to the device. I'm basing this on the RC values shown -- it looks like there's a time constant of around 32us, and the inputs for this device are rated only up to 400 ns/V at 4.5V operation (which I assume you are operating at due to the input voltage of 4.54 V).

    The application report Implications of Slow or Floating CMOS Inputs explains the reasoning behind this in detail.