Digital Power Supplies Are Getting Easier

I've been watching the product releases from several power supply IC manufacturers, with a specific interest in devices classified as digital controllers.

To me, this new breed of highly integrated devices shows where power supply designs are going (or should be) for all but the simplest point of load (POL) power supplies.

The classic servo loop uses a couple of power FETs, one or more op-amps, one or more comparators, a voltage reference, and the usual collection of resistors, capacitors, and an inductor. With higher levels of integration, designers can now be provided a lot more functionality to play with.

The new digital power supply controllers combine a conventional analog servo control topology with a digital controller based on a microcontroller or a DSP. This is a high level of integration compared to the classic version and far more than you'd want to design and build discretely. Different manufacturers blend the analog and digital portions in different amounts. Some are easier to design with than others.

Microchip has just released the MCP19111. With this device, Microchip has made the digital portion less dominant. That makes the design process a lot less scary.

The IC will find use as a synchronous buck POL controller. POL implies that the device is placed in the immediate vicinity of the load circuitry; “controller” means you add the power FETs, which are controlled by the IC, rather than having the FETs on the IC itself. Synchronous controllers have two power FETs. The upper FET drives current into the inductor and causes the magnetic field to build. The lower FET allows the current to continue flowing in the inductor as the magnetic field decays. Of course, they are not both on simultaneously. The duty cycle or ratio of on-time (upper vs. lower) sets the output voltage.

The onboard microcontroller makes it easier to adjust the output voltage, as well as the trip-point of the output current, via firmware. You can monitor for conditions such as overtemperature and output over- or undervoltage (and report them as needed). You can also set up one power supply to track the voltage of another (a form of power supply sequencing) or soft start (where the output voltage ramps up at a specified rate).

You initially configure the device using Microchip's MPLAB® X integrated development environment software. Once that's done, you use the PMBus or I2C bus to communicate with and modify the MCP19111.

Two other very nice features: The IC can be powered from a single 4.5-32V supply, so it will see use in designs where 5V is available and in industrial systems where there is a 24V intermediate bus. And the operating temperature range is -40 to +125°C, which again makes it useful in industrial applications and especially in automotive applications.

I would be remiss if I did not mention one aspect of this device that some design engineers will surely find concerning. The output voltage is set purely by the firmware. There is no classic two-resistor divider network to set the operating point. This certainly adds to the convenience; if design requirements change and you need a slightly different supply voltage for your microprocessor or FPGA, you can just make a quick change to the firmware. But this also raises concerns about possible failure modes (or screwups). What if someone changes the code inadvertently — or intentionally, for that matter? This makes it pretty easy to blow up an expensive microprocessor or FPGA. I hope Microchip will address this concern at some point.

The '19111 controller is available in a QFN package, 28-pin 5mm X 5mm. The cost is less than $3 for each '19111 controller and 28 cents each for the recommended Microchip power FETs in 5,000-unit quantities. Since Microchip also makes the FETs, it makes sense to use them, because you know these devices will work properly with the IC. Other FETs will likely work, too, but you'll need to do more validation before your design is complete.

3 comments on “Digital Power Supplies Are Getting Easier

  1. SunitaT
    March 9, 2013

    @Brad, thanks for the post. The MCP19111 integrates a configurable peak current mode analog controller, synchronous MOSFET driver, compensation networks, a 8bit Microcontroller. This gives MCP19111 significant configurability to adjust analog compensation, power supply controllers.

    March 12, 2013

    Nice IC. With the digital control, it could also look at the control of the synchronouse FET (bottom). If the output power requirement drops, turning off the sync-FET drive can help improve efficiency. Furthermore, adjusting the switching frequency (similar to skipping, or burst mode) improves the low-power efficiency. These ware some of the PWM concepts I was working on years ago – before I left the semiconductor industry.

  3. Brad Albing
    March 12, 2013

    Quite right – there are tricks you can like PFM vs PWM or the diode emulation technique, tho' the latter is for conventional buck switchers' lower FET, not so much for flyback switchers.

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