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Texas Instruments CD74ACT151ME4 - Failure

Steve Mitropoulos
Prodigy 100 points

Other Parts Discussed in Thread: CD74ACT151

The part that we have experienced some failures with is the Texas Instruments CD74ACT151ME4.  I believe that the failures are due to exposure to a condition that violates the specification, however, circuit boards that have been built with an alternate source and have been exposed to the same condition have not experienced the failures.
The alternate source is Fairchild 74ACT151SC.

The condition that I believe is causing the failure is the board has lost power (No VCC), but the input signal is still present (5 VDC).
The specification sheet for the TI part states that Vih maximum is Vcc, so we are violating the specification for the condition where Vcc = 0V, but Vin = 5V.
The specification sheet for Fairchild states that Vih max is Vcc + 0.5V, so we are also violating that specification for the given condition.

The results appear to be different between the manufacturers.  No Fairchild parts have failed when exposed to this condition, but the Texas Instruments parts have failed almost every time when exposed to this condition.  The part overheats when power is restored, then fails.  The component cracks in some instances.

I understand that the component is being exposed beyond the specification limits and we are experiencing latch-up.  We are also taking steps to protect against this condition (inserting series resistors on the inputs to keep the input current very low).

My question/concern is, why the Texas Instrument part is so much more sensitive than the Fairchild part, and is this possibly a case of the component being a counterfeit part?

  • 0
    TI__Guru 71526 points

    Steve,

    If you look on page 2 of the CD74ACT151 datasheet, yes, the ABS MAX is Vcc, however, footnote (2) states this can be exceeded if the IK is below the limit.  There are latch-up diodes on the inputs to this device, and I'm guessing the Fairchild device also has an ESD cell (diode) that is why it is spec's at Vcc + 0.5v. 

    If they are adding the series resistors, then this will prevent the excess current, which is most likely destroying the input circuit.

    Why are they questioning if the devices are counterfeit?  Did they buy them through distribution channels, or a broker?  The other possibility is that there has been some previous damage due to ESD from improper handling or storage.  Perhaps you should initiate a Counterfiet Request action.

    -Leonard