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PMBus – What the Heck Is It?

In a previous blog, Digital Power Supplies Are Getting Easier, where I discussed digital power supplies, space did not permit a discussion of a related topic, the PMBus (Power Management Bus). While not exactly a proper analog topic, it's worth taking a quick look at the PMBus, so please permit a brief foray into the digital realm.

The PMBus is a fairly simple, low-speed bus (around 400kHz, top speed) that is used to allow smart power supply subsystems (with either digital or analog control loops) to communicate with one another. Since analog design engineers work with power supplies, it seems to be a topic for which it is worth spending a few minutes.

According to the PMBus standards organization:

The Power Management Bus (PMBus) is an open standard power-management protocol with a fully defined command language that facilitates communication with power converters and other devices in a power system. The protocol is implemented over the industry-standard SMBus (System Management Bus) serial interface and enables programming, control, and real-time monitoring of compliant power conversion products.

Additional information from the standards site mentions that leading power supply and semiconductor companies were involved in creating the bus standard. You would therefore see the bus used both on an Eval board from a semiconductor manufacturer and from an OEM of power supplies (e.g., the “silver box” supply). For a list of PMBus adopters, click here.

You would be more likely to see the PMBus used on a digital power supply (i.e., containing a digital control loop) than an analog supply, but only because the digital supplies offer more control capability in general and because the designers are already working in the digital world. Thus, adding a communications link between supply subsystems is a logical extension of the existing design circuitry.

Here are a few details just to give an overview:

  • Similar to I2C — low cost
  • Electrically compatible with I2C, mostly
  • But more robust than I2C
  • More features than I2C

The screen-shot below of a slide from a presentation from System Management Interface Forum shows a bit of detail. The bus structure looks familiar:

As noted, the bus can zip along at 400kHz (maximum), so that is probably fast enough to monitor for over- or under-voltage conditions; and over-current conditions. You can also sequence power supplies and adjust output voltages as needed. Note that you can't do cycle-by-cycle current limiting of your SMPS, but that is not what the PMBus was designed for.

So, as an analog design engineer, that's mostly all you need to know. Have you designed with the PMBus? Let us know about your experiences below.

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10 comments on “PMBus – What the Heck Is It?

  1. Brad Albing
    April 19, 2013

    More info that I did not know. Thanks, Scott.

  2. SunitaT
    April 22, 2013

    The protocol is implemented over the industry-standard SMBus (System Management Bus) serial interface and enables programming, control, and real-time monitoring of compliant power conversion products.

    SMbus is the foundation for the PMbus. Does it mean that physical layer of SMbus and Pmbus's architecture is same and only data layer is modified? Can the existing SMbus be replaced by PMbus without any hardware changes?

  3. amrutah
    April 23, 2013

    SunitaT,

       PMBus uses SMBus interface to communicate with other devices in the system.  SMBus uses I2C interface which is usually 400kHZ operation and good and reliable communication protocol.  You can check these data sheet where SMBus interface is used by PMBus.

    LM25066 power management & protection IC with PMBus.

    MAX8688 It is a digital power-supply controller with PMBus.

    Hope this helps.

  4. amrutah
    April 23, 2013

    Brad, Thanks for sharing this.

       PMBus uses the interface of the SMBus, wherein it can communicate to other devices regarding whether it is hitting a current limit or power limit, fault detection like short circuit, in built temperature sensor to communicate on chip temperatures.

  5. Davidled
    April 23, 2013

    When I overview diagram, I think that there is extra addres to read/write data from memory with control signal. May be if CANBus protocol is used, control signal line may be elminated and other type units might be used without physical address. CANBus is mostly used in the Automotive Industry. 

  6. amrutah
    April 24, 2013

    The SMBus uses a I2C protocol wherein any device can become a master device and starts sending the command (and address as well).  The address bus is an internal register address and I dont think it is used for communication.

    I dont know anything about the CANBus, Thanks for sharing this.

  7. Davidled
    April 24, 2013

    Brief Overview of CANBUS

    CANBUS has a message-based protocol with different baud rates including 250Kbps with J1939 Protocol specification. This protocol usage is getting grown in the other field including medical industry.  I am wondering if this protocol can be applied to power management system.

  8. Brad Albing
    April 24, 2013

    More good info – thanks for some links to specific devices.

  9. Brad Albing
    April 24, 2013

    We'll se if anyone else can jump in with further info on CANbus.

  10. Brad Albing
    April 30, 2013

    That's how I recall bus usage with the devices we sold at my previous job.

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