While the switching power supply remains one of the last pure analog systems in captivity, the need for intelligent operation is forcing designers to add digital electronics. The latest piece of evidence to support this claim is the Power Management Bus (PMBus), a clever digital communication scheme that networks multiple power supplies with the system processor. Based on the venerable I2C port, the PMBus provides a well-defined protocol that enables an entire range of new system capabilities such as remotely programmable system settings, operating modes, system data logging, firmware program updates and much more. This is a stark contrast to the present power supply communication channel (that is, an orange wire called "Power Good"). Make no mistake about it - PMBus will forever change the way system power management is implemented, and your next design won't be competitive if it isn't PMBus enabled.
"So what is the best way to implement PMBus in my system?" you might ask. Good question. The answer is: a small inexpensive MCU is the fastest, easiest and most economical path to PMBus design. For example, a product like the C8051F300 contains a high-speed CPU, ample FLASH memory and an I2C port. It's packaged in a super tiny 3 x 3 mm package and sells for about the price of a pair of drivers - just add code. Great!
But adding even a little cost to a supply design is a problem. "How do I minimize the added cost of an MCU?" you might ask. Another good questionand the answer: amortize the MCU cost by incorporating more functionality into it! For example, consider a common windowed fault detector based on a dual comparator, four resistors and a couple of bypass caps for a total of seven components, one of which is an 8-pin IC. Now, multiply this number times the number of these detectors in a typical supply (maybe three or four). Now, consider this: a few lines of MCU code will replace each of these hardware detectors saving board space and cost. In fact, the savings in these components alone may come close to covering the cost of the MCU. But waitthere's more - the MCU can also implement soft-start, intelligent fault recovery and management functions like external fan speed control. And after all this, it will still have enough horsepower to implement additional functions that may be needed later.
Where else can you get so much for so little? The MCU not only implements PMBus in all of its glory, but also reduces system parts count and saves cost and space. AND, it adds the flexibility to easily change system functionality over the PMBus at any time. Not bad, eh?
Don is Director of Applications Engineering for Silicon Labs and you can reach him at Don Alfano.