Take Control of Your Power Supply

Bruce Haug, Linear Technology


There is a growing trend designs in using a digital communications bus

Did you ever feel like it’s time to take control of your power supplies? If not, you should, since there is a growing trend to configure and monitor power supplies over a digital communications bus. This is frequently referred to as “Digital Power” or “Digital Power System Management” (DPSM). This technique enables designers to simplify and accelerate system characterization, optimization and data mining during prototyping, deployment and field operation.

Knowing the condition and operating status of a voltage regulator is perhaps the last remaining “blind spot” in today’s modern electronic systems, since they normally do not have the means for directly configuring or remotely monitoring key operating parameters. Nevertheless, it can be critical for reliable operation that a regulator’s output voltage drift over time or during an over temperature condition be detected and acted on before a potential failure event occurs. A DPSM approach to such a system can monitor the performance of a voltage regulator and report back on its health so that corrective action can be taken prior to it going out of specification or even failure.

DPSM enables users to act upon the information collected from the load and the system with the following benefits:

•          Faster Time-to-Market

a.         Change power parameters without re-spinning the PCB

b.         Perform quick system characterization, optimization and data mining

•          Load-Level Benefits:

a.         Control power supply accuracy over time and temperature

b.         Margining to test FPGA tolerances

c.          Increase system efficiency by load shedding

•          System-Level Benefits:

a.         Digital access to board level power diagnostics

b.         Monitor and pinpoint system wide power consumption

c.          Fault Management/Fault Logging

•          Data Center Benefits:

a.         Power consumption trends, detect fluctuations and changes over time

b.         Develop predictive analytics to minimize operating cost

c.          Make energy management decisions

The interface

Emerging DPSM products tend to support configurability and monitoring via a 2-wire interface such as PMBus, an open standard I²C-based digital interface protocol. This provides a means for DPSM products to seamlessly integrate with existing embedded systems and architectures, Board Mount Controllers (BMCs) and Intelligent Platform Management Interface (IPMI) functions. For simplicity and ease of use, especially at the earliest stages of hardware development and testing, it is common to interact with DPSM devices through a graphical user interface (GUI) running on a PC and through a USB-to-PMBus communications converter tool (commonly called a dongle). The GUI can provide control and monitoring of key operating parameters such as power consumption, voltages, sequencing, margining and even fault log records.

The PMBus command language was developed to address the needs of large multirail systems. In addition to a well-defined set of standard commands, PMBus compliant devices can also implement their own proprietary commands to provide innovative value-added features.  The standardization of the majority of the commands and the data format is a great advantage to OEMs producing these types of system boards. The protocol is implemented over the industry-standard SMBus serial interface and enables programming, control and real-time monitoring of power conversion products. Command language and data format standardization permits easy firmware development and reuse by OEMs, which results in reduced time-to-market for power systems designers. For more information, see http://pmbus.org. With over 75 PMBus standard command functions, users can take full operational control of their power system using one of the most popular open standard power-management protocols. 

When DPSM Makes Sense

Digital control over analog power supplies with a simple PC connection is especially valuable during the development stage to get systems up and running quickly. There can be as many as 30 point-of-load (POL) voltage rails and the system designer needs to be able to easily monitor and adjust supply voltages, sequence supplies up/down, set operating voltage limits and read parameters such as voltage, current and temperature as well as access detailed fault logging. High accuracy is extremely important in these systems to maintain tight control over the rails while achieving maximum performance. 

Specifically, in data centers, a key challenge is to reduce overall power consumption. This can be done by rescheduling the usage of underutilized servers and enabling the shutdown of other servers based on power drawn. To meet these demands, it is essential to know the power consumption of the equipment. DPSM can provide the user with power consumption data, allowing for smart energy management decisions to be made.

Consider the example of protecting expensive ASICs from the possibility of an overvoltage condition. High-speed comparators must monitor the voltage levels of each rail and take immediate protective action if a rail exceeds safe operating limits. With a DPSM, the host is notified when a fault occurs via the PMBus alert line and dependent rails can be shut down to protect the load. Achieving this level of protection requires excellent accuracy and very fast response times.

DPSM is being adopted because of its ability to provide accurate information about the power system and its ability to autonomously control and supervise many voltages. Linear Technology has several digital power products, including the LTM4676.

The LTM4676, a DPSM µModule Regulator

The LTM4676 is a dual 13A or single 26A μModule (micromodule) step-down DC/DC regulator with a digital interface, enabling system operators to remotely command and supervise a system’s power condition and consumption. The device is comprised of fast, dual analog control loops, precision mixed-signal circuitry, EEPROM, power MOSFETs, inductors and supporting components housed in a 16mm x 16mm x 5mm BGA (Ball Grid Array) package.  In addition to delivering power to a point-of-load, the LTM4676 features configurability and telemetry monitoring of power and power management parameters over a PMBus.

The LTM4676’s 2-wire serial interface allows outputs to be margined, tuned and ramped up and down at programmable slew rates with sequencing delay times. Input and output currents and voltages, output power, temperatures, uptime and peak values are readable. To evaluate the performance of the LTM4676 and other Linear Technology DPSM products it is necessary to download the LTpowerPlay GUI and obtain a USB-to-PMBus converter.

With +/-1% maximum DC output error over temperature, +/-2.5% current readback accuracy, integrated 16-bit delta-sigma ADC and EEPROM, the LTM4676 combines best-in-class analog switching regulator performance with precision mixed signal data acquisition. The LTM4676 operates from a 4.5V to 26.5V input supply and steps-down VIN to two outputs ranging from 0.5V up to 5.4V. Two channels can current share to provide up to 26A (i.e. 13A+13A as one output). As many as four LTM4676 devices can current share to generate an output  of up to 100A. Custom configuration of the EEPROM contents is not required. At start-up, output voltages, switching frequency, and channel phase angle assignments can be set by pin-strapping resistors. Figure 1 shows a typical application using the LTM4676 as a dual 13A μModule regulator with DPSM for control and monitoring purposes.

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Figure 1. Dual 13A μModule Regulator with DPSM for Control & Monitoring Purposes

There is often a need to margin several rails to specific voltages and to check the voltage levels of each after margining step. This process is simplified and quickened with DPSM. Figure 2 shows how the output voltages of one LTM4676 µModule regulator reacts to a 7.5% margin low PMBus command. The nominal 1V output goes to 0.92V and the nominal 1.8V output goes to 1.66V. Expanding this feature up to 72 rails is possible through LTpowerPlay enabling a much easier margining process and verification of voltage settings.

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Figure 2. Output Voltage Readback Using DPSM of the LTM4676, VOUT Margined 7.5% Low

The LTM4676 has the capability to power each of the two onboard DC/DC converters with its own dedicated input voltage. Figure 3 shows input current fluctuation of these two input voltage sources. This feature allows for power consumption data to be obtained to monitor trends, detect fluctuations and changes over time to make energy management decisions.

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Figure 3. Input Current Readback Using DPSM of the LTM4676 for Real-Life Applications

It is not uncommon for a system board to have over 30 power rails. These types of boards are usually densely populated and the DPSM circuitry cannot take up too much space. Furthermore, it must be easy to use and be able to control a high number of rails. Such solutions must operate autonomously or communicate with a system host processor for command, control and to report telemetry information.

Linear Technology's LTM4676, LTC2977, LTC2974, LTC3880, LTC3883 can be combined to control up to 72 voltages on a single segment of an I2C bus. The LTM4676 and LTC3880 manage and generates up to two high current rails. The LTC3883 manages and develops a single high current output. The LTC2977 manages up to eight rails, and the LTC2974 manages up to four rails.

Figure 4 shows how a multi-rail system can be controlled with various Linear Technology µModules, managers and DC/DC controllers. These rails normally have strict requirements for sequencing, voltage accuracy, overcurrent and overvoltage limits, margining and supervision.

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Figure 4. Block Diagram for Control of 19 Rails via an I2C/PMBus

DPSM creates a tool for system designers to control power supplies with a simple PC connection and digital interface. This capability is valuable during the development and debug stage, enabling designers to get their systems up and running quickly with the ability to control and adjust supply voltages, limits and sequencing. Margin testing is easier since the entire test can be controlled by a couple of commands over an I2C/PMBus.

DPSM can provide the user with power consumption data, allowing for smart energy management decisions to be made, which can reduce overall power consumption. Power system data can be sent back to the OEM about the power supplies health, effectively opening up the blind spot with regards to a DC/DC converters well. A regulators output voltage drift over time or an over temperature condition can be detected and acted on before a potential failure event occurs. If a board is returned, the fault log can be read back to determine which fault occurred, the board temperature and the time at which the fault happened. This data can be used to quickly determine root cause, or if the system was operated outside of its specified operating limits or to improve the design of future products. For high rail count systems and OEM’s that want to take control of their power systems, Digital Power System Management is a powerful tool.