Stress exceeding the levels prescribed in absolute maximum ratings specifications may lead to chip malfunctions. Key word: may.
Within the last decade, I was the head of a captive tier 1 automotive embedded electronics department for a global vehicle supplier. Our job was two-fold: on the one hand to build electronic units for our own vehicles whenever an external tier 1 could not meet our price and time-lines, and on the other hand to support external tier 1 companies as well as our own vehicle engineers to deliver robust and first-time right solutions.
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Our vehicle engineering team was in the process of doing final testing of a prestigious export consignment of buses destined for a UN peace-keeping mission. A initial batch of two hundred buses were undergoing on-road tests when we discovered that around twenty of them were exhibiting electronic cabin climate control subsystem malfunctions.
This particular subsystem had been developed by a globally reputed tier 1 supplier. Their engineering team was promptly summoned to troubleshoot and fix the problem. Unfortunately, the initial troubleshooting progressed for two weeks without any desired outcome. Finally, we as in-house electronics experts were asked to intervene and rescue the seemingly intractable situation.
Their team leader described the associated circuit block that was a suspected culprit as follows:
A three terminal low-dropout regulator (LDO) with a fourth enable pin is used to power the climate control logic. Whenever we wish to reduce drain on the battery by turning off the climate control system, the LDO is disabled by deactivating the fourth enable pin. Unfortunately, this LDO is misbehaving in all the twenty malfunctioning buses. Their “enable pin” always remains disabled internally, shutting off the output!
“What is your diagnosis?,” we asked. “Your vehicle environment is full of transient spikes reaching up to 70 volts,” he countered. “We have tied the enable pin through a resistor to the 24 V battery bus. No wonder the LDO is refusing to work, since its rating is exceeded.”
“You need to clean-up your vehicle transients to ensure the health of our system,” he advised, showing us a report issued by a certified laboratory. “Our control unit has passed the automotive transient burst tests as per international automotive transient norms. If our design was erroneous, our unit should have failed during the transient test.”
In summary, according to him, our vehicle was inflicting worst transients than those prescribed in automotive test transient specifications. I went through the supplier’s schematic, along with the LDO datasheet. The latter document clearly indicated that the absolute maximum rating for the enable pin was 45V DC. Like all datasheets, however, it also cautioned engineers that any stress exceeding the levels prescribed in the absolute maximum rating may lead to chip malfunction.
I pointed out the datasheet note, explaining to him that a transient suppressor in his circuit was needed to limit external transients to below 45V. My team immediately set to work, installing external transient suppressor units in each bus so that the consignment could be released overnight. But the supplier engineers were not convinced, repeatedly pointing out the claimed “passed” conclusion from the test laboratory.
Automotive global transient test norms specify an acceptance criterion as follows:
- The unit should first pass an in-advance functional test
- The unit can now undergo a “transient burst” test that bombards the power bus with spikes as high as 150V
- The unit should then again pass the same functional test as prior
Note, however, that an absolute maximum rating of 45V is applicable to the worst-case rated LDOs in the field. In contrast, the majority of the chips withstand much higher voltages during operation. This explained why a majority of the buses did not suffer from the malfunction. When a supplier submits samples to the laboratory, test agencies do not test “violation of absolute maximum rating”. They only apply the acceptance criterion in terms of successful functional tests both pre- and post-transient test.
But the supplier engineering team was not prepared to accept above argument. “If you don’t agree with us, let’s meet again tomorrow. This time, please also bring with you the LDO supplier’s application engineer. Both of you should declare in one voice that your circuit does not violate absolute maximum ratings. We have no time to argue now; we need to expedite corrective measures overnight.”
The next morning, we met again with the the climate control supplier engineer, this time also including the semiconductor application engineer in the discussion. The semiconductor engineer confirmed our understanding, much to the dismay of the supplier engineer. Our buses were happily dispatched to their destination after adding necessary protection units and are running without problem to this very day.
Let me summarize the lessons and insights from this case study, which I also frequently share with my automotive clients and trainees:
- An absolute maximum rating of, say, 45 volts does not mean that all chips would get destroyed at 46 volts and beyond. That said, other chips’ operating life may, however, still be reduced.
- Understand the limitations of engineering tests based on visual observations of correct functionality for electronic units. The unit may be violating datasheet limits, ratings, operating conditions etc., but may still seem to be working flawlessly.
- An accurate way to ensure robust and flawless behavior across a mass-produced population of units is to record voltage and other electrical signatures in a laboratory for key circuit points. Doubly ensure that the same is not violating any data sheet limits.
- It is good engineering practice to jointly audit key circuit blocks with the assistance of authorized chip application engineers. Most semiconductor companies are happy to do so, since it preserves their field reputations. They will also gladly prescribe proactive measures to strengthen circuit designs in order to avoid subsequently facing “field surprises” such as this incident.
Vishwas Vaidya is a graduate of the Indian Institute of Technology in Delhi, India. Currently, he is self-employed as an engineering consultant and industry faculty member in the field of embedded systems for global automotive clients and high-repute academic institutions. Vishwas’ articles and research reports have appeared in many worldwide engineering publications.
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