Automatically Compensated PoL Controller IC Integrates PMBus Compatibility

The MAX15301 is an innovative, PMBus compliant, mixed-signal power management IC with a built-in high-performance digital PWM controller for point-of-load (POL) applications. Based on Maxim’s patented InTune™ technology, it automatically compensates the digital PWM control loop.

 

Designers of point-of-load converters will get a big assist from the new MAX15301 controller IC from Maxim. MAX15301’s digital power technology performs an automatic compensation routine based on measured parameters, enabling construction of an internal mathematical model of the power supply, including external components. The result is a switching power supply that achieves excellent dynamic performance with guaranteed stability. Furthermore, this power supply model enables several proprietary algorithms that optimize efficiency across a wide range of operating conditions.

Design of the MAX15301 keeps its operational environment in mind. Its “out-of-the-box” operation enables fast prototyping. This allows rapid development of the power supply subsystem before completing board-level systems engineering. And, its pin-selectable settings establish its functional hardware configuration.

This IC relies on mixed-signal design techniques to control the power system efficiently and precisely. It does not require any software to configure or initialize the device. The MAX15301 can regulate and perform power management tasks without any programming. Using standard PMBus commands, its functions can be monitored and optimized through the SMBus interface, resulting in ease of design and flexibility.

Among the IC’s features are:

  • High effective loop bandwidth to minimize external component requirements.
  • Integrated power conversion and power management.
  • Optimal partitioning of the digital power management and the digital power conversion domains to minimize start-up times and reduce bias current.
  • 4.5V to 14V input range with efficient on-chip power rail generation for self-bias.
  • 0.5V to 5.25V output range with ability to start with a pre-biased output.
  • Remote output differential voltage sensing.
  • Output voltage sequencing, tracking, and margining.
  • Selectable switching frequency from 300 kHZ to 1MHz
  • Protection against output overvoltage and overcurrent.
  • Accepts resistor-set parameters that enable “plug-and-play’ operation before any PMBus commands are written.
  • Supports over 60 standard and manufacturer specific PMBus commands.

 

Predictive Loop

Fig. 1 shows a typical operational circuit using the MAX15301. Outwardly it appears similar to many other controller IC circuits. Inwardly, however, it is different from any of its predecessors.

The MAX15301 uses an intelligently predictive loop rather than a passively reactive loop, such as PID-based digital power devices. Using advanced algorithms, the MAX15301 automatically compensates a point-of-load controller with advanced power management features. Its predictive model technique uses an internal model of the control loop, including the external L-C filter. It makes measurements at start up, so it can always predict what the next duty-cycle should be and it follows that prediction, which essentially reduces phase lag. The control surface (or law) is continuous over the whole operating range and it accounts for duty cycle (D) saturation (0% < D <100%). This prevents the usual difference between small and large signal response with a buck converter.

Intune History
The basis for the InTune patents was IP for a switch mode power supply (SMPS) that measures external parameters and uses the information to modify the compensation. The fundamental patents were purchased from the University of Toronto by Maxim. The method for doing the parametric extraction and for using state-space control is covered in IP Maxim acquired through their purchase of L&L Technology in 2009. Maxim further refined and created patents associated with implementing this IP in silicon with low power dissipation and in a small die area as they collectively designed the MAX15301 and other ICs.

 

The InTune patent portfolio now exceeds a couple of dozen patents related to fundamental ideas down to detailed IP associated with developing commercial products. The control technique allows closed-loop calibration throughout regular converter operation. SMPS parameters, such as output capacitance and load, are estimated automatically by the IC using an internal sine wave generator to inject various sine waves in the loop and reading them back with internal A/D converters that probe the loop in multiple locations. The extracted signals are used to set up a series of matrix equations that are solved by the internal MCU to come up with the optimal compensator using time-domain criteria.

The Toronto patent applications were applied for in April 2008. The patents were issued in September and November 2009. Maxim then purchased the patents and started developing the necessary circuits, converting the concepts to an actual IC with PMBus functionality.

A copy of the original patents by Prodic and Zhao are on page 47 of the April 2011 Power Electronics Technology issue. It is also available at the Power Electronics Technology website athttp://powerelectronics.com/power_management/april-2011-patents-0411/.copy.

 

The InTune compensator’s control law is valid over the complete control surface. This eliminates the need for users to determine and set thresholds associated with transitioning from linear to non-linear modes. Additionally, the InTune controller is the only PWM controller that accounts for duty-cycle saturation effects. Unlike competing PID controllers (see glossary: PID Controllers), the InTune controller uses a feedback analog-to-digital converter (A/D) that digitizes the full range of output voltage, eliminating the compromise associated with “windowed” A/D converters used in other controllers. For more on InTune, see the sidebar “InTune History”.

The control loop is separated from the housekeeping, power monitoring and fault management blocks. Control loop parameters are stored in an on-chip non-volatile flash memory. An internal microcontroller enables monitoring operating conditions via its SMBus interface. The PWM control loop is implemented using hardware state-space machines. There is no DSP or MCU in the loop. This partition minimizes power consumption while optimizing performance. In addition, this advanced digital controller implementation reduces the number of output capacitors needed to meet system requirements, resulting in more efficient, smaller footprint, cost-effective solutions.

State-Space Model

The MAX15301 inherently has sufficient stability margins and good transient performance under a wide range of operating conditions and with a wide range of output capacitors. In operation, the MAX15301 automatically constructs an internal state-space model (State Estimator) of the control plant (Fig. 2). Its internal model gives access to state control variables that are otherwise unavailable. The state control variables are used to set the proper compensator values. For a given input-to-output step down ratio and PWM switching frequency the MAX15301 sets the compensator coefficients for that application. Upon start-up or in response to a PMBus command, the MAX15301 performs the InTune calibration. During this calibration, it measures several power train parameters whose extracted values create the internal model to optimize the converter’s bandwidth and transient response. Having the additional state variables enables more accurate and predictive control, resulting in better dynamic performance and reduced requirement for output capacitors.

Fig. 3 shows the MAX15301’s simplified controller implementation using a digital non-linear state space (NLSS) model predictive controller, an adaptive gate drive circuit, PWM, PLL based master timing generator, a microcontroller unit (MCU), and a SMBus serial communication port with PMBus protocol.

The NLSS compensator generates the duty-cycle command for the DPWM (Digital PWM) block. The DPWM block generates the required PWM outputs for the driver.

The proprietary NLSS control algorithm achieves fast transient response without any external loop-compensation components. The internal gate driver drives a pair of external n-channel power MOSFETs for optimum efficiency.

The MAX15301 uses the digital pulse width modulation control to regulate the output voltage. Traditional PWM regulators (both analog and digital) use classical control methods for dc-dc converters based on linear models of a discrete time nature and root locus, Bode and Nyquist plots. These linear time-invariant approximations work well for small signals. However, when large transients cause duty-cycle saturation, the performance of the closed loop can be degraded (larger overshoots) and the output transients will be “slower” (large settling times). Tighter regulation performance during these disturbances is now a requirement. The MAX15301 addresses the issue by using model-predictive based non-linear feedback design to compensate the PWM. Fig. 4 is an example of the MAX15301’s response to a step function compared with the results obtained with a PID controller.

The MAX15301 also features a high-speed serial port (HSSP) that enables real-time, non-invasive evaluation of the control loop dynamics using a Maxim supplied GUI known as the PowerToolTM. Both frequency and time-domain loop analysis can be performed without the need for costly lab equipment. This enables rapid design verification over the full range of expected operating conditions.

Power Component Selection

Power component selection for the Maxim InTune products follows the typical Point of Load design methodology. The inductor is selected to supply sufficient output current with an acceptable current ripple and efficiency. The input capacitors are selected to provide acceptable current draw on the input power supply during high current transient events. The FETs are selected to provide maximized efficiency and board area. However, with optimized InTune technology, the output capacitors can be minimized to provide a specified output voltage ripple while still achieving good transient response. The InTune technology can also be highly optimized when coupled with all ceramic output capacitors.

MOSFET Driver

The IC includes an advanced, high-efficiency driver for an external MOSFET gate drive. This driver features adaptive non-overlap timing to optimize efficiency. The algorithm continuously adjusts the high-side and low-side timing to improve efficiency over the full range of voltage, current and temperature variation. In addition, the IC can automatically adjusts gate drive voltage based on output current, which optimizes efficiency over its full-load range. Also featured is a discontinuous switching mode to improve light-load efficiency.

The InTune technology includes a small, single inductor, dual output switching supply that provides regulated voltages to internal circuits such as the driver supply. The dual supply is achieved with a single 0806-sized inductor to maximize overall efficiency and minimize board space. This inductor can be omitted if the efficiency gain is not required; the internal supplies will then operate as linear regulators.

PMBus Compatibility

An on-board PMBus compliant serial bus enables the MAX15301 to communicate with a system supervisory host controller for monitoring and advanced fault management. A full suite of power management features enables designers to eliminate complicated and expensive sequencing and monitoring ICs from their designs.

The MAX15301 monitors input voltage, output voltage, output current and both internal and external temperatures. The fault thresholds and responses are factory set but may be changed using PMBus commands. Fault detection can be individually enabled or disabled for the parameters through PMBus. Default warning and fault limits are factory-set. The response to a fault condition can be changed through PMBus.

PMBus compatibility includes many of the standard PMBus commands. A PMBus 1.2- compliant device uses the System Management Bus (SMBus) version 1.1 for transport protocol and responds to the SMBus slave address. Here, SMBus refers to the electrical characteristics of the PMBus communication using the SMBus physical layer. The MAX15301 employs five standard SMBus protocols (Write Word, Read Word, Write Byte, Read Byte and Send Byte) to program output voltage and warning/faults thresholds, read monitored data, and provide access to all manufacturer-specific commands.

The MAX15301 is available in a 32-lead, 5mm x 5mm TQFN-P package and operates over the -40°C to +85°C temperature range. Its Pb-free/RoHS-compliant package has an exposed pad for added thermal management. The IC will shutdown if its junction temperature increases beyond 125 °C. A PMBus command can lower this threshold.

Note: The MAX15301 is subject to a license from Power-One, Inc., related to digital power technology patents owned by Power-One, Inc.

Glossary: PID Controller
PID, proportional-integral-derivative controllers are a widely used control structure. A PID controller calculates an error value as the difference between a measured process variable and a desired set point. The controller attempts to minimize the error by adjusting the process control inputs. In operation, “P” depends on the present error, “I” on the accumulation of past errors, and “D” is a prediction of future errors. The weighted um of these three actions is used to adjust the process via a control element, such as a power supply output voltage.

 

State Space Control

A state space representation is a mathematical model of a physical system as a set of input, output and state variables related by first-order differential equations. To abstract from the number of inputs, outputs and states, the variables are expressed as vectors, and the differential and algebraic equations are written in matrix form. This is a convenient way to model and analyze systems with multiple inputs and outputs.

Model Predictive Control

These controllers rely on dynamic models of the process, often linear empirical models obtained by system identification. These models predict change in the dependent variables of the modeled system caused by changes in the independent variables. Dynamic characteristics difficult for PID controllers include large time delays and high-order dynamics.

 

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