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SN74HC08: broke down

Gary Zeng
Intellectual 1195 points
Part Number: SN74HC08
Other Parts Discussed in Thread: SN74HCS08

Tool/software:

Hi expert,

My customer is using SN74HC08DR in VFD product and they found three IC broke down when they are under the factory test.

The issue is that pin 2 of SN74HC08 in above schematic is always 0.1V, which will cause the output of pin3 to low voltage level.

They test the impedance between pin1 and pin2 twice, which are 5.23kohm/5.21kohm when IC broke down and the impedance is 6.2Mohm/22.1Mohm when IC is good.

 

They also did ABA test and new device can fix the issue but issue IC indeed broke down.

Could you please help review the schamtic and find the reason why it broke down.

  • 0
    Guru 317370 points

    What is connected to SC1, OC1, RS, and R81?

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 1195 points

    Hi Ladisch，

    SC1 and OC1 are fault indication signals. Normally they are 5V voltage level and they will be 0V when issue appears.

    RS is reset signal from watchdog. When the machine powers on, RS wiil be 0V for 600ms and then pulled high to 5V. 

    R81 is 100ohm resistor. output signal of U8 will go through RC filter(R81 and C249) then go intoU14 pin1. I add the schmatic as below:

  • 0 in reply to Gary Zeng
    Guru 317370 points

    100 Ω limits the output current to 50 mA, which exceeds the absolute maximum ratings and could damage the device. Better use 1 kΩ and 10 pF. (But I do not think that this caused your problem.)

    The input signal at pin 2 has slow edges that exceed the Δt/Δv limit. You should use the SN74HCS08 instead. (But I do not think that this caused your problem.)

    Damage is usually caused by over/undervoltage, probably at the damaged pin itself. I do not see an obvious way how this could happen here (there is a resistor between the LOCK2 signals and pin 2), so maybe one of the other signals has ESD or transient voltages. (Is it possible for a human to touch anything? Are there connectors that are (un)plugged?)