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© linear technology Application Notes | August 17, 2015

High current rails demand a converter with control & supervision

The increasing complexity in large computing systems has exerted pressure on power supply manufactures to improve efficiency, accuracy and transient response.
More recently however, they are also demanding digital control for monitoring, settings and reporting. High efficiency is essential in distributed systems, where high step-down ratios from intermediate voltage busses are used to create local low voltage supplies sourcing high currents in order to minimize thermal concerns by poor conversion efficiencies. Host sys­tems can have dozens of local voltage rails delivering a wide range of power levels. In datacom systems, for example, there can be as many as 50 point-of-load voltage rails some of which can have currents up to and exceeding a hundred amps. As a result, system designers would like to be able to easily monitor and adjust supply voltages, sequence, 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 “Power System Management (PSM)”, and enables designers to control, monitor and supervise dozens of rails in real time. The ability to digitally change power supply parameters significantly 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 PSM 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 PSM 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 PSM devices through a Graphical User Interface (GUI) running on a PC and through a USB-to-PMBus communications conversion tool commonly called a dongle.

PSM can be used to monitor the performance of a remote 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. PSM also 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

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 SMBusTM 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 on this topic, please visit http://pmbus.org.

PSM is being adopted due to its ability to provide accurate information about the power system and its ability to autonomously control and supervise many voltages. Linear Technology has several PSM products to address these needs and we continue to introduce new parts on a regular basis.

New PSM DC/DC Controller

The LTC3882 is a recently released dual channel DC/DC synchronous step-down PWM controller with PMBus-compliant serial interface. It operates with input power bus voltages from 3V to 38V and each channel can produce independent output voltages from 0.5V to 5.25V. Up to four LTC3882s can operate interleaved in parallel, creat­ing single-output rails containing up to eight phases with currents as high as 40 amps per phase.

Multiples of 6- or 8-phase designs can also be developed when power or reliability dictate higher phase counts. Once its onboard EEPROM is programmed, the LTC3882 can operate autonomously without host support, even during fault conditions. Figure 1 shows a typical LTC3882-1 application schematic.


Figure 1. LTC3882 Typical Applications Schematic

Internal Architecture

In order to support high step-down ratios and fast load transient response, the LTC3882 utilizes a constant frequency, leading-edge modula­tion voltage mode architecture. This archi­tecture is combined with a very low offset, high bandwidth voltage error amplifier and internal feed-forward compensation. Internal feed-forward compensation instantaneously adjusts the duty cycle for changes in input voltage, significantly reducing output overshoot or undershoot during a transient event.

Both channels feature remote output volt­age sense to compensate for the voltage drop associated with long PCB traces. A separate control loop yields exceptional DC and dynamic multiphase load sharing when outputs are paralleled. Figure 2 shows the tran­sient response of the figure 1 schematic with a 15A step-load. The maximum deviation from the nominal output voltage is less than 25mV.

Selection of Power Stages

Each LTC3882 channel provide selectable PWM control protocols for interfacing to power stage designs that have 3.3V-compatible control inputs. The user can choose the optimum type of power stage for the design requirements: discrete FET drivers, DrMOS devices or power blocks. These can be mixed and matched on a per channel basis, allow­ing optimization of power subsystem partitioning, size and cost, according to the power delivery needs of each rail.

Leading-edge modulation affords fast, single-cycle response to output load steps and does not have a restriction on the minimum duty cycle. PWM output control pulses can become very small with this scheme for high step-down ratios, and the minimum on-time is normally limited by the power stage design, not the control­ler. For the most compact solution it is possible to use only ceramic output capaci­tors and the LTC3882 features program­mable active voltage positioning (AVP), allowing further optimization of equivalent series resistance (ESR) and reduction in output capacitor size.

Depending on the needs of the applica­tion, peak efficiency or solution size can be prioritized by choosing an opti­mal operating frequency. The LTC3882’s programmable switching frequency of 250kHz to 1.25MHz supports optimiza­tion of inductor size and output current ripple. The LTC3882 can also serve as a shared PWM clock master or accept an external clock input for synchronization to another system time base.


Figure 2. Transient Response of Figure 1 Circuit
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Figure 3. Efficiency and Power Loss Curves for a 12V input to 1V Output
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Figure 4. Dynamic Load Balancing During an Output Transient
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Low DCR Sensing for High Power

At relatively high output currents, con­version efficiency must be maximized to limit heat production and minimize related cooling costs due to conduction losses. It is important to minimize the power loss in the current sense element to maximize efficiency because it continuously sees the full DC load current plus additional ripple current. The LTC3882 supports conventional sense resistor topologies as well as low DCR sense schemes that can produce only a few tens of millivolts. The fixed ramp voltage mode PWM architecture allows large signal control of the duty cycle and eliminates noise concerns that could be created by low DCR designs using current mode control schemes. The efficiency and power loss are shown in Figure 3 represented by the figure 1 schematic.

The LTC3882 contains an optional digital output servo function. When enabled, the 16-bit ADC output for channel volt­age is used to servo to the desired average output value. In this case, the converter has an impressive typical output error of only ±0.2% and a worse case error over temperature of ±0.5%. For multiphase single output operation, the LTC3882 features a separate current sharing loop that provides accurate load balancing, an improvement over conventional voltage mode converters. The output channels are desig­nated as control masters or as slaves by pin strapping.

The IAVG pin on the master channel provides a voltage analog of its instantaneous output current. A filter capacitor of 100pf to 200pf is added to this line, which is then routed to all slave phases. The slaves use this infor­mation and the primary COMP control voltage from the master to match their own output current to that of the master. Figure 4 shows that this matching is maintained dynami­cally through high speed load steps.

Accurate telemetry

The LTC3882 monitors critical supply parameters with an internal 16-bit ADC. Digital readback via PMBus is available for input and output voltages, output currents, duty cycles and temperatures. The LTC3882 tracks, maintains and pro­vides peak values for these parameters.

Beyond basic supply parameter telemetry, the LTC3882 can report a wide range of internal and external status information to the system host over the PMBus.

Fast Programmable Fault Response

Faults can be detected and communicated using a shared fault bus between LTC3882s as well as other Linear Technology PSM family members, such as the LTC3880. The LTC3882 provides a standard open-drain ALERT output for notification of a wide range of fault conditions to the bus host. The LTC3882 implements high speed, low level hardware responses to critical faults to protect the power stage and down­stream system load. PMBus commands can then be used to configure higher-level responses, mask faults to the system, and determine which faults are propagated to the shared fault bus. This provides flexibil­ity in dynamically managing fault handling at the system level, even after hardware has been designed and fabricated.

The LTC3882 includes extensive log­ging capability that records the state of converter operating conditions imme­diately prior to a fault. This log can be enabled and stored to internal EEPROM to provide a black box recorder function for in-system diagnosis or subsequent remote debugging of abnormal events.

Advantage of PSM

There are many reasons to consider use of a PSM controller. PMBus commands can be issued to the LTC3882 to set output volt­age, margin voltages, switching frequency, output on/off sequencing and other operating parameters. In total, the LTC3882 supports over 100 PMBus commands, both standard and custom. A principal benefit of this programmability is reduced design cost and faster time to market. Once a fundamental hardware macro design is complete, many variations can quickly be created, brought to operation and verified as needed by simply adjust­ing digitally programmable parameters inside the LTC3882 controller.

Adjustments can continue beyond production release as needed, including fully synchronized re-sequencing/retiming of power rails. Combined with optional external resistor programming of key supply parameters, this kind of flexibility can avoid risky, costly PCB spins or hand-wired modi­fications due to last-minute changes in requirements or evolving system use.

Final configurations can be readily stored on its internal EEPROM using a variety of means, including custom factory program­ming. Once a configuration is stored, the controller powers up autonomously to that state without burdening the host for additional programming. However, even after a final EEPROM configuration is loaded, optional external programming resistors can be used to modify a few key operating parameters such as the output volt­age, frequency and phase assignments.

Once designed, the multiple address­ing schemes supported by the LTC3882 allow the system to communicate with devices globally or selectively at the rail, device or individual channel level, depending on control and monitoring requirements. PMBus then facilitates sophisticated high level system operations, such as energy-efficient application load balancing in high current requirements. These functions would simply not be cost-effective or even possible with conventional power supply components in large systems.

Conclusion

PSM is a way for system designers to control power supplies with an existing system host processor or with a simple PC connection. 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. For high rail count systems, some of which require high current and for OEM’s that want to take control of their power systems, PSM is a simple, practical and powerful tool.
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Author: Bruce Haug, Senior Product Marketing Engineer, Power Products at © Linear Technology Corporation
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