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Digitally Enhanced PowerAnalog DC/DC Controllers forPoint-Of-Load ApplicationsIn many systems microcontrollers are used to control multiple Point-of-Load (POL) DC/DC convertersforming a hybrid control system to manage system start-up behavior, monitor electrical parameters andmanage the power consumption of peripheral sub-systems. The most sophisticated solutions, however, canbe found on motherboards of computers, graphic cards or on CPU blades of servers, where VoltageRegulator Modules (VRM) directly communicate with their load to adjust supply voltages and/or even adapttheir control characteristic to temporary operation conditions. This kind of “intelligent” power conversionmanagement and control offers significant advantages in terms of total system efficiency, performance andreliability – preferences which are dominant in industrial, medical, automotive and consumer marketsegments as well. Andreas Reiter, Technical Business Development Manager for PowerElectronics, Europe, Microchip Technology

Since almost one decade MicrochipTechnology focuses on so-called intelligentrespective smart power conversion(IPC/SPC) applications to bring morefeatures and enhanced capabilities to powerconversion applications. One main focushas been, and still is, the fully digital controlof power converters/inverters using DSP-based microcontrollers with highlydedicated high-speed, high resolutionperipherals as well as dedicated controllerfamilies for hybrid control systems

combining microcontrollers with fully analogbased control loops. Looking into theparticular solutions more closely, it becomesobvious, that neither the analog-basedhybrids nor the fully digital solutions are 100% analog or 100 % digital. Both urgentlyneed their analog and digital counterpart toovercome certain limitations and thereforeoffer specific strength for their specific targetapplications. The latest product family ofsmart power conversion controllersMCP191xx marks a new technological step

in making power converters more intelligentand focuses on dedicated power convertertopologies and applications.The first members of this family of

products MCP19111 are merging ananalog synchronous buck convertercontroller and an 8-bit MCU into onemonolithic IC, offering enhanced, flexibleconfiguration, control and monitoringcapabilities as well as they allow theintegration of standardized or proprietarycommunication to align multipleconverters within a higher powermanagement structure. The MCP19111are fully programmable in C-language,offering full flexibility to adjust the device tomany different application requirements, toadapt to certain operating conditions, toimplement generic monitoring tasks andcustomized features. The wide inputvoltage range of 4.5 V to 32 V DC andoutput voltages as low as 0.5V togetherwith drivers supporting up to 2 A source/4A sink opens a wide variety of applicationsto be supported.

Digital enhancementsHaving a digital controller on the same dieas the analog switching regulator makes itpossible to dovetail analog functions anddigital control very tightly enabling directmanipulation of the compensation circuit,switching frequency, dead-time control,system level thresholds and many otherfeatures during runtime. Furthermore, asthe MCU itself is encapsulated in the

Figure 1: The MCP19111 merges an analog synchronous buck converter controller and an 8-bit MCUinto one monolithic IC

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analog switching regulator architecture, theneed for additional auxiliary power suppliesor external MOSFET drivers has beeneliminated. Figure 1 shows a high-levelblock diagram of the MCP19111 DigitallyEnhanced Power Analog Controllertogether with a typical application circuit.The analog switching regulator sectioncovers all components of the analogcontrol loop including the MOSFET driversand also contains the auxiliary supply forthe MCU. The digital section consists of an8-bit PIC12F mid-range MCU core with 8kB of Flash and 256 byte of RAM. It furtheroffers up to 15 GPIOs, of which eight areadditional analog inputs, an I2C/SMbusbased serial communication interface,external interrupts and three free timers.Many internal signals like the input voltage,output voltage or the inductor current canbe monitored directly on-chip withoutneed for external sensing. The digitalimplementation even allows reading thecurrent duty ratio, a very useful featurewhich was, up to now and for manytechnical reasons, reserved to fully digitalcontrollers only.Besides the enhanced monitoring

capabilities the monolithic integration of adigital core also gives direct access tomany parameters, which are typically fixedin hardware or inaccessible in Silicon. Themost remarkable ones are the adjustabledead-time, the programmablecompensators, internal feedbackcalibration, programmable protectionthresholds and even the capability toswitch over between current and voltagemode control during runtime.

Adjustable dead-bands In synchronous buck converters the dead-time setting between high- and low-sideswitch has significant influence on the totalefficiency of the system. If analogcontrollers offer any adjustable dead-timesettings at all, the designer has to refer tosome sort of worst case scenarioconsidering the highest temperatures andload conditions, where the dead-timetypically needs to be at maximum, and“program” this value in hardware e.g. byplacing capacitors and/or resistors. Thisinevitably results in increased core anddiode losses as the converter will mostprobably be never exposed to these

assumed worst case conditions.One proper solution would be to

automatically adapt the dead-time tocertain load and temperature conditions.Unfortunately using an on-board zero-crossing detector to always drive theswitches with an optimum dead-time hascertain serious limitations for two reasons.First, all kinds of analog zero crossingdetectors are based on comparators. Thefastest (affordable) analog comparatorshave typical propagation delays of 15-20ns, which is, given the results shown inFigure 3, too slow to reach the optimumlevel. Second, this zero-crossing detectorwould have to operate in the switchingnode of the half-bridge where the highfrequency switching noise would requirefilters, which would slow down the triggereven more, eventually making this featureeffectless.However, where the analog domain

fails, the digital domain offers a solution.The most common technique used toaccomplish this optimization is to monitorand analyze the converter’s outerconditions until they become stable. Assoon as a steady state operation isdetected, the dead-time is modified andthe duty ratio of the high-side switch ismonitored. The theory of this technique forconstant voltage converters is that understeady state conditions, the shortestrelative on-time of the high-side switchdetermines the point of highest efficiency,as in this point the least amount of powerhas to be drawn from the bus to provide acertain, constant output power (see Figure 2).Figure 3 shows results of one single

sweep over a defined range of dead-timesettings during steady state operation,measured on a test bench. The green lineshows the dead-time applied to the risingedge of the high-side switch (DTR). Thered curve gives the development of theon-time of the high-side switch over thedifferent dead-time settings, the dottedblack line shows its 3rd orderapproximation. The given range for the dead-time

sweep was determined by characterizingthe system and defining a best-casescenario (shortest dead-time) and a worst-case scenario (longest dead-time). Thesweep was performed with the maximumresolution of 4 ns at 90 % load (Vin = 12V, Vout = 3.3 V, Iout = 9 A). On the leftside of the chart, the duty-cycle starts withvalues of around 1.394 µs and rapidlydrops as soon as the dead-time isincreased. In this area the high- and low-side switch already show some overlapand some of the power drawn from theinput is directly shorted to ground.At dead-times around 25 ns the on-time

Figure 2: MCP19111 high-level application block diagram

Figure 3: Basic equations of the Automated Dead-Time Adjustment (ADTA) technique

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reaches its minimum of 1.384 µs and startsto rise again when the dead-time is furtherincreased. In a non-adjustable design, thedead-time would have been adjusted to atleast 70 ns for the switches used, so thetypical on-time for this operating conditionwould have been 1.395 µs. In accordancewith equation III of Figure 2, the differencebetween the original and optimized high-side on-time is 11 ns wide. On the firstglimpse this doesn’t sound much, but in thishigh frequency converter it stands for ~0.9% increase in efficiency respective anincrease in total efficiency from ~92 % to93 %.

Adjustable switching frequency andcompensation networkOne additional very nice feature is thecapability to adjust the compensationnetwork and switching frequency bysoftware. This not only eases the basicadjustment during setup configuration, italso allows to make adjustments duringruntime. This, so far, has been theunbreakable core domain of fully digitalcontrollers. In hard-switching topologieslike synchronous buck converters, theswitching losses are dominantlyresponsible for the majority of powerlosses. To improve the efficiency especiallyat light loads, reduction of the switchingfrequency can significantly help to improvethe total efficiency of the converter. However, when the switching frequency

is being reduced while the compensationnetwork is fixed in hardware, commonlythe gains start to drop and might result in aloss of phase and gain margin. Tocompensate this effect, an adjustment ofthe system’s gains is required. TheMCP19111 offers registers to adjust theramp voltage of the PWM generator, thezero frequency (resonance frequency atthe origin, defining the first pole), the totalgain and slope gain as well as the slopeitself. Further there are register sets toadjust the amplifier offset and currentsense gain. Although this technique mightrequire a process of extensive systemcharacterization, it bares the chance forsignificant improvements in terms ofefficiency and stability.

Very light load efficiency optimizationIn asynchronous buck converters thepower losses in the freewheeling diode aredetermined by the forward voltage droptimes the current. As the significant forwardvoltage drop across the diode is presentpermanently and cannot be minimized, anadditional switch with significantly lowerforward voltage drops is commonly usedto bypass/replace the diode eventuallyforming a synchronous rectifier. Thistechnique is commonly used when load

currents above 1A are required. However,during light load conditions, when there isvery little current flowing through the low-side switch, the power required to drive itsgate exceeds the savings gained bybypassing the freewheeling diode with aswitch. To support further efficiencyenhancements under this particularconditions, the MCP19111 offers the so-called diode-emulation mode, where,when enabled, the low-side driver isturned off. With a disabled driver, the gateis not biased anymore and the MOSFET’sbody diode will become the rectifier,minimizing the power losses.This kind of measure to minimize partial

power losses and increase the total overallefficiency can additionally be backed-up byusing Microchip’s new family of powerMOSFETs MCP870xx. This family of lowRDS(on), well balanced Figure-Of-Merit(FOM) fast Power-MOSFETs offer a range

of different On-Resistance vs. Total GateCharge (QT) combinations to optimize thetotal FOM of the half-bridge. The higher QTof the low side switch, the more effectivebecomes the diode emulation mode ofthe MCP19111.

Optimizing no-load operationThe MCP19111 are current modecontrollers to offer best performanceduring normal operation. However, acurrent mode controller needs to have atleast some current flowing to functionproperly. When load switch into low-powerstandby operation, the output of theconverter still has to provide the nominaloutput voltage, but the output power mightalmost be zero. Commonly current modecontrollers switch into some sort of hick-upor Pulse-Frequency-Mode (PFM) operationwith higher output ripples, violating lineregulation tolerances and often also

Figure 4:Measurement resultsof one sweep overdifferent dead-timesettings during steadystate operation

Figure 5: Graphicaluser configurationinterface

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causing serious EMI issues. To overcomethis limitation of typical current modecontrollers, the MCP19111 can beswitched into pseudo voltage modecontrol by disabling the current loopresulting in a improvement of the outputvoltage and system stability.

Usability and tool chainThe list of enhanced features of theMCP19111 offers a number of options toconfigure and optimize the system. Theopen programmability of the MCU addseven more degrees of freedom. ThereforeMicrochip offers the Graphical UserInterface (GUI) shown in Figure 4, whichcan be used to make certain adjustmentsand configurations without the need towrite code. This GUI works with an open-source firmware, which can be modifiedand be used as a template to developindividual, more advanced features.In addition to the configuration interface,

Microchip offers a second GUI for testpurposes, enabling the user to directlycommunicate with the device using thePMbus protocol (see Figure 5). This GUI isworking with Microchip’s PICkit SerialAnalyzer (Part-Number DV164122), which isa generic low-cost USB-to-UART/SPI/I?C

interface and can be directly used to monitorand debug the device during operation.

ConclusionsAlthough most of the techniques areknown and some features can also befound on already existing parts, theMCP19111 family of devices opens a newchapter in the history of intelligentswitching regulators. The density of

dedicated features together with the freeprogrammability makes the powerfuldifference. In the world of smart powercontrollers, the MCP19111 brings analogand digital control schemes closer togetherand removes existing limitations by makingall features available and accessible toengineer and therefore forms the base ofinnovative, efficient and reliable highperformance POL converters.

Figure 6: MCP19111 PMbusTM test interface

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