Freescale v. too many others
One of my pet peeves is when a datasheet lists 'typical' values with no min or max. I’ve written about this a couple of times: here and here. It seems like this problem is getting worse as components get more complex. In fact, one FAE admitted that “typical” specs are often driven by marketing’s needs, not engineering analysis.
Sometimes the results are almost laughable. TI, which generally does a good job of characterizing their devices, has a step-down converter (the TPS82740A) whose Iq is rated at 360 nA typ, 2300 max. That’s a huge range of values. One can only conclude that the typ number is meaningless. (Not to knock the part – this is a very innovative converter. It will stop switching and connect Vin to Vout through a FET if Vin is roughly at the desired output level. Very cool.)
Maximum values are especially rare in the domain of ultra-low power MCUs. Vendors are in a real slugfest to prove their parts have lower sleep currents than the competitors, and too often “typ” is the only rating given for this critical parameter. Here’s a “typical” example from a vendor of an ARM MCU who makes a big production about their low-current specs:
The careful designer is left scratching his head, with no idea what sort of results he’ll see in a real-world application.
Hats off to Freescale. Perusing the datasheet for their KL02 Cortex M0+ MCU I came across the following statement:
“The maximum values stated in the following table represent characterized results equivalent to the mean plus three times the standard deviation (mean + 3 sigma).”
Wow! A year or so ago I asked several semi-vendors what ‘typical’ means and none could define it. Here’s a concise mathematical model that makes a lot of sense. Three sigma means you can be sure 99.7% of the parts will not exceed the listed values. 'Typical' still is not defined, but cautious engineers really only care about max values.
A portion of the table that the three sigma rating applies to follows:
It’s interesting that typ and max are generally pretty close together. If typ is the mean – which is only a guess – then the standard deviation they experience is pretty tiny. They have extremely good control of their manufacturing process.
Other members of their Kinetis family are well characterized as well. Some, like the KL03, sip very gently from the power supply in deep sleep modes.
It’s tempting to advertise the best possible numbers for important specs, and ‘typical’ results are a lot more compelling than worst-case values. But these are aspirations, not guarantees, and we engineers can’t design to some marketing person’s fantasy. I hope more companies follow suit.
Jack G. Ganssle is a lecturer and consultant on embedded development issues. He conducts seminars on embedded systems and helps companies with their embedded challenges, and works as an expert witness on embedded issues. Contact him at email@example.com. His website is www.ganssle.com.