In my travels, I always get hit with many unique problems, situations and company politics. Like the famous TV detective Columbo (see Figure 1) you have to try to find the “root cause” of the problem in spite of all the noise. Recently, I had a few skirmishes with absolute maximum ratings (AMR) where there were hiccups after many years of successful electronic circuit operation.
The definition of absolute maximum ratings (AMR) is listed below to refresh your memory.
Absolute maximum rated voltage: The maximum voltage that may be applied to a device, as listed in its data sheet and beyond which damage (latent or otherwise) may occur; device manufacturers for a specific device and/or technology frequently specify it.
“What are the Factors used to determine AMR4”?
Listed below are the dominant factors as well as device qualification testing and simulation.
- Semiconductor Fabrication Process
- Design Rule Limitations
- Competitor’s Second Source advantage
None! And integrated circuits (ICs) are not fortune tellers.
An IC's absolute maximum rating is the limit of the conditions under which it may be operated. Operation beyond these limits will damage it and may destroy it.
How far beyond the limits is never stated. Some devices are very robust, some are not, but no manufacturer will provide support for deviation from the limits. The only safe rule is to treat "never" as never. But understanding why exceeding absolute maximum limits can cause damage allows us to design better systems5”
Watch Out For Temperature Ratings of AMR
“Unless otherwise specified, an ambient temperature of 25 ± 3 °C is assumed for all absolute maximum ratings6”
An example of AMR temperature dependence is the breakdown voltage (BVDss) of a MOSFET as shown in Figure 2.a & b at -50°C V(BR)DSS which is about 90% of the 25°C maximum VDSS.7” and for the 60 Volt MOSFET about 77.5 volts at 200°C.
Review Manufacturer’s AMR notes on Complex Mixed Signal Switching Regulator parts in Chip Scale Packages (CSP)
As an example, the TPS6265x device is a high-frequency synchronous step-down dc-dc converter optimized for battery-powered portable applications. The Absolute Maximum Ratings table in the device data sheet specifies dc limits to voltages or currents that may be applied to the device pins. (See Figure 3).
“For the SW node (see Figure 4), the maximum voltage rating refers to the maximum voltage that can be applied before the oxide layer in the low-side MOSFET silicon begins to break down and causes a short between the drain and source. However, the negative voltage rating for the SW node refers to the parasitic p-n junction in the low-side MOSFET, which forms its body diode. During switching operation, the negative rating is exceeded due to the operating principles of a synchronous buck converter. This does not violate the absolute maximum rating, because the current causing the voltage excursion is applied by the output inductor during switching operation. In the case of a source other than the output inductor, the -0.3-V voltage rating should never be exceeded. A dc source, if supplying a voltage lower than -0.3 V, could theoretically source infinite current through the body diode, which could damage the MOSFET7”
Watch out for Thermal AMR on miniature CSP
Take care with the thermal AMR requirements for miniature CSPs. These devices can respond quickly to thermal transient loads and trigger the threshold thermal shutdown temperature, which can cause havoc to troubleshooting your design.
Finally, a large percentage of your problems and failures can be avoided by reviewing the AMR from the data sheet. When in doubt contact the manufacturer’s application engineer but as always double-check any advice.
Figure 2a - Normalized Breakdown Voltage (BVDss) of a MOSFET versus Temperature