Control & supervision of multiple voltage-rails

Bruce Haug, Linear Technology


A popular way to control a high rail count system is over a digital communications bus

As the number of voltage rails continues to grow inside systems due to higher complexity and proliferation of digital content, it is essential to have a mechanism to monitor and control these rails. Often times there can be as many as 50 point-of-load voltage rails and system designers need to be able to easily monitor and adjust supply voltages, sequence supplies up/down, set operating voltage limits and read parameters like voltage, current and temperature as well as access detailed fault logging.

A popular way to control a high rail count system is over a digital communications bus. This is frequently referred to as “Digital Power” or “Digital Power System Management (DPSM)”, and enables designers to control, monitor and supervise dozens of rails. The ability to digitally change power supply parameters reduces time-to-market and down time by eliminating what would have historically required physical hardware, circuit, and/or system bill-of-material modifications.

Emerging DPSM products tend to support configurability and monitoring via a 2-wire interface such as PMBus, which is 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 and Intelligent Platform Management Interface functions. For simplicity and ease of use, especially in the early 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.

One of the last remaining blind spots in today’s modern electronic systems is the condition of the power supply, since they do not normally have the means for directly configuring or remotely monitoring key operating parameters. It can be critical for reliable operation that a regulators output voltage drift over time or an over temperature condition be detected and acted upon before a potential failure event occurs. With DPSM 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 a failure. DPSM allows users to act upon the information collected from the load and the system with the following benefits.

Faster time-to-market

• Change power parameters without re-working the PCB

• Perform quick system characterization, optimization and data mining

 Load-Level Benefits

• Control power supply accuracy over time and temperature

• Margining to test FPGA tolerances

• Increase system efficiency by load shedding

System-Level Benefits

• Digital access to board level power diagnostics

• Monitor and pinpoint system wide power consumption

• Fault management/fault Logging

Data Center Benefits

• Power consumption trends, detect fluctuations and changes over time

• Develop predictive analytics to minimize operating costs

• Make energy management decisions

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 allows for easy firmware development and reuse by OEMs, which results in reduced time-to-market for power systems designers. For more information, go to 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. 

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 real time.

DPSM is being adopted rapidly 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 to attain this and the LTC3882 is one that has recently been released.

DPSM DC/DC Controller

For example, the LTC3882 is a dual-output multiphase synchronous step-down DC/DC Controller with a serial digital interface that uses leading-edge modulation voltage mode control for excellent transient response, and an ability to operate with very low DCR (0.25 milliohm) inductors allowing for higher efficiency and up to 40A per output. Compatible power train devices include DrMOS, power blocks or discrete FET drivers and N-channel MOSFETs.

The digital interface enables system designers and remote operators to command and supervise a system’s power condition and consumption. The ability to digitally change power supply parameters reduces time-to-market and down time by eliminating what would have historically required physical hardware, circuit, and/or system bill-of-material modifications.

The LTC3882’s 2-wire serial interface allows outputs to be margined, tuned and ramped up and down at programmable slew rates with time-based or voltage-based sequencing delay times. Input and output currents and voltages, output power, temperature, uptime and peak values are readable. The device is comprised of fast, dual analog control loops, precision mixed-signal circuitry, EEPROM and is housed 6mm x 6mm QFN-40 package.

To evaluate the performance of the LTC3882, the LTpowerPlay GUI (free to download), USB-to-PMBus converter and demo kits are available. With +/-0.5% maximum DC output error over temperature, +/-1% current read back accuracy, integrated 16-bit Delta Sigma ADC and EEPROM, the LTC3882 combines excellent analog switching regulator performance with precision mixed signal data acquisition.

The device operates from a supply input voltage ranging from 3V to 38V and produces an output voltage between 0.5V and 5.25V. Two channels can accurately share current to provide up to 80A. Up to four LTC3882s can operate in parallel for 2-, 3-, 4-, 6- or 8-phase operation. At start-up, output voltages, switching frequency and channel phase angle assignments can be set by pin-strapping resistors or loaded from internal EEPROM. Figure 1 shows a simplified LTC3882 schematic which using DrMOS as the power train devices and delivers 1V @ 80A from a 12V nominal input.

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Figure 1. Two Phase Single Output LTC3882 Applications Schematic

Output Voltage Margining

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 LTC3882 configured for two outputs 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 allowing for a much easier margining process and verification of voltage settings.

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Figure 2. Output Voltage Read back, Using DPSM of the LTC3882,  VOUT Margined 7.5% Low

Digital Power System Management 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 control a high rail count. Such solutions must operate autonomously or communicate with a system host processor for command, control and also report telemetry information.

Linear Technology's LTM4676, LTC2977, LTC2974, LTC3882, LTC3883 can be combined to control up to 72 voltages on a single segment of an I2C bus. The LTM4676 and LTC3882 manage and generate up to two high current rails. The LTC3883 manages and develops a single high current output. The LTC2977 manages up to 8 rails, and the LTC2974 manages up to 4 rails.

Figure 3 shows how a multi-rail system can be controlled with various Linear Technology µModule regulators, 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 3. Block Diagram for Control of 19 Rails via an I2C/PMBus

A useful tool
DPSM provides a mechanism for system designers to control power supplies with a simple PC connection or an existing system host processor. 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 without the need for physical hardware, circuit, and/or system bill-of-material modifications. 

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 being.

A regulator’s output voltage drift can be detected over time 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 occurred. 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.