I have a customer with finished board inventory on hand with the LMC555 installed. The design of the board was originally done with the LM555 in mind. Due to an oversight the LMC555 was sourced and installed instead of the LM555. The issue of concern is that the applied supply voltage of the circuit is beyond the absolute maximum figure indicated in the data sheet for the LMC555, by a little bit.
The data sheet for the LMC555 lists +15V as the absolute max supply voltage. On the customers board the 555 timer is powered from three 5.1V Zener diodes stacked in series with a 1Kohm limiting resistor to a Vin of 24VDC +/-5%. The total nominal Zener voltage is 15.3V however, they are 5% tolerance parts so their worst case nominal voltage is ~16V. These figures do not take into account temperature induced variation in the Zener voltage. The operating ambient temperature range the part is expected to see is 0 to 60C. The systems are powered 24 hours per day 7 days per week and are expected to be in the field for up to 7 years.
The customer has requested further information regarding the level of expected reliability and the risk of failure to assist in making a decision whether to rework existing inventory. Hoping you can offer insight on the following with regard the LMC555:
- Is there any margin on the absolute maximum voltage listed in the data sheet (i.e., can the component operate above 15V without performance degradation)?
- What feature on the chip degrades when the absolute maximum supply voltage is exceeded?
- What is the mechanism that causes that degradation?
- Are there any equations to model that degradation?
- Is there any accelerated reliability test data in which the supply voltage was greater than 15V.
- How does the chip ultimately fail when it fails for excessive supply voltage.
This last question is critical because the 555 timer controls the enable line of the main switching power supply (a National Semi LM22676 set at 5.8V). If the 555 timer OUT pin is high when it fails, then the board will be held in a power off state.
If it would be helpful I have the customers schematic showing this portion of the design which I can forward if required via confidential channels. Many thanks for your time on this.
