Why choose digital power supply

introduction

Experienced digital power users are generally well aware of the benefits of digital power system management. However, for those who are considering whether digital power makes sense for their products, the benefits of digital power may not be so obvious. Typical questions they ask include: Will our time to market be longer when adopting digital power? How difficult is it to adopt digital power? What is the learning period? Are there additional costs? Will our customers value digital power? Will this technology open up new markets? Will we fall behind if we don't adopt digital power in our product line? They need to know the answers to these questions to help decide whether adopting digital power is a good choice for their end application.

Why use digital power?

Digital control of analog power supplies with a simple PC connection is particularly useful during the development phase, when designers need to get the system up and running quickly. There may be as many as 30 point-of-load (POL) voltage rails, and users need to be able to easily monitor and adjust supply voltages, sequence power-up/down, set operating voltage limits, and read parameters such as voltage, current, temperature, and obtain detailed fault log data through the digital interface. In such systems, high accuracy is extremely important to maintain tight control of the voltage rails and achieve maximum performance.

In the data center, a key challenge is to reduce overall power consumption by rescheduling workflows and moving jobs to underutilized servers, shutting down other servers. To meet such requirements, it is important to know the power consumption of end-user devices. A properly designed digital power management system can provide power consumption data to the user, allowing intelligent energy management decisions to be made.

In today’s modern electronic systems, the regulator’s environment and operating status is perhaps the last “blind spot” that needs to be understood, as there is usually no way to directly configure or remotely monitor the regulator’s critical operating parameters. Being able to detect regulator output voltage drift over time or overheating and take action before a potential failure event occurs is critical to reliable operation. A well-designed digital power system can monitor the performance of the voltage regulator and report whether the regulator status is normal so that corrective action can be taken before it is out of specification or fails.

To protect expensive ASICs from possible overvoltage conditions, high-speed comparators must monitor the value of each voltage rail and take immediate protective action when a voltage rail exceeds the specified safe operating limit. In a digital power system, when a fault occurs, the host can be notified through the PMBus alert line, and the related voltage rail can be shut down to protect the powered device such as the ASIC. To achieve such protection, excellent accuracy and very fast response time are required.

Digital power management is adopted because digital power can provide accurate information about the power system and can easily control and monitor dozens of voltages autonomously. It can be very difficult to probe and monitor 30 load point voltages everywhere on a complex system board. System designers do not have to write a line of code unless they need the host processor to read the telemetry data and perform simple fault intervention. Obviously, manufacturers need to provide customized and affordable devices for specialized markets, which can be easily used by both novice and experienced users. Linear Technology offers several digital power products, and the recently launched LTC3880 is one of them.

Digital power is the right choice

The LTC3880 and LTC3880-1 are dual-output high efficiency synchronous step-down DC/DC controllers with an I2C-based PMBus interface for digital power system management. These two devices also provide best-in-class analog switching regulator performance and precise mixed-signal data conversion to facilitate power system design and management. The LTpowerPlay software development system supports these two devices, which has an easy-to-use graphical user interface (GUI).

The LTC3880 / LTC3880-1 allows digital setting and readback of real-time control and monitoring of critical point-of-load converter functionality. Programmable control parameters include output voltage, margining and current limit, input and output check limits, power-up sequencing and tracking, switching frequency, and identification and traceability data. Built-in precision data converters and EEPROM allow regulator configuration settings and telemetry variables, including input and output voltage and current, duty cycle, temperature, and fault logs to be captured and stored in nonvolatile storage.

The LTC3880/-1 features an analog control loop for optimal loop stability and fastest transient response, without the quantization effects common in slower digital control loops. These devices can provide two independent outputs or be configured to provide a two-phase single output. Up to 6 phases can be interleaved and paralleled to achieve accurate current sharing between multiple ICs, thereby minimizing input and output filtering requirements for high current and/or multi-output applications. An integrated amplifier provides true differential remote output voltage sampling, resulting in highly accurate regulation that is not affected by board IR drops. Figure 1 shows a typical application that uses the LTC3880 to generate 1.8V/20A and 3.3V/15A from a 12V bus voltage.

Figure 1: LTC3880 Application Schematic

Using Linear Technology's graphical user interface-based LTpowerPlay development software, the LTC3880/-1 configuration is easily saved to the internal EEPROM through the device's I2C serial interface. On-chip memory allows specific user settings. In addition, these controllers can power up autonomously without burdening the host processor. Default settings for output voltage, switching frequency, phase, and device address can be optionally configured by external resistor dividers. The LTC3880/-1 has a built-in 16-bit ADC, providing best-in-class programmability and telemetry readback.

Setting the LTC3880/-1 Resolution and Telemetry Accuracy

LTC3880/-1 Programming Resolution

VOUT Command

• 12-bit resolution

• 5.5V range, 1.375mV per step

• 2.75V range, 687uV per step

Current limit set point

• 3-digit resolution, ±5mV accuracy

• 25mV to 75mV range

OV/UV VOUT Monitor

• 8-bit resolution, ±2% accuracy

OV/UV VIN Monitor

• 8-bit resolution, ±2% accuracy

LTC3880/-1 Telemetry Resolution

Input Current

• 16-bit resolution

• Each phase and total

VIN

• 16-bit resolution, ±2% accuracy

VOUT

• 16-bit resolution, ±0.5% accuracy

IOUT

• 16-bit resolution

• ±1% accuracy at 6mV VSENSE

• Calibration factor for DCR

The LTC3880/-1 has built-in integrated MOSFET gate drivers to drive all N-channel power MOSFETs in the input voltage range of 4.5V to 24V, and these devices can provide ±0.50% accuracy in the output voltage range of 0.5V to 5.5V with an output current of up to 30A per phase over the entire operating temperature range. The LTC3880/-1 can also drive power components or DR MOS devices. The minimum on-time of the LTC3880/-1 is only 90ns, making it very suitable for compact high-frequency/high step-down ratio applications. Accurate timing and event-based sequencing across multiple chips allows optimization of power-up and power-down of complex multi-rail systems.

The LTC3880 features a built-in LDO. The LTC3880-1 allows the use of an external bias voltage for maximum efficiency. Both devices are available in a thermally enhanced 6mm x 6mm QFN-40 package with an operating junction temperature range of -40°C to 105°C.

Digital Systems Management for Real-World Applications

Large multi-rail power boards typically consist of an isolated intermediate bus converter that converts 48V, 24V or other backplane voltages to a lower intermediate bus voltage (IBV), typically 12V, which is distributed throughout the PC card. Each point-of-load DC-DC converter steps down the intermediate bus voltage to the required rail voltage, typically in the 0.6V to 5V range, with currents ranging from 0.5A to 120A. Figure 2 shows how a multi-rail system can be controlled using a variety of Linear Technology controllers and DC/DC converters. The point-of-load DC/DC can be a self-contained module, a monolithic device, or a solution consisting of multiple DC-DC controller ICs with inductors, capacitors, and MOSFETs. These voltage rails typically have stringent requirements for sequencing, voltage accuracy, overcurrent and overvoltage limiting, margining, and supervision.

Figure 2: Block diagram of how to control 15 voltage rails via one I2C/PMBus

Clearly, the complexity of power management is increasing, and boards with more than 30 rails are not uncommon. Such boards are densely packed, and digital power system management circuitry must not take up too much board space. The circuitry must be easy to use and capable of controlling a large number of rails. Linear Technology’s LTC2978 combines all the functions necessary to work with the LTC3880/-1 and LTC2874 to control up to 72 voltages on a single I2C bus segment. The LTC3880/-1 controls, monitors and generates two high current rails. The LTC2978 controls and monitors up to eight rails, while the LTC2974 controls and monitors four rails. Such solutions must either operate autonomously or communicate with the system host processor to obtain commands, implement control, and report telemetry data. Figure 3 shows one channel of the LTC2978 controlling a DC/DC converter.

Figure 3: LTC2978 controlling an external DC/DC converter

The PMBus command language was developed to meet the needs of large multi-rail systems. In addition to a well-defined set of standard commands, PMBus-compliant devices can also use their own proprietary commands to provide innovative value-added features. The standardization of most commands and data formats is beneficial to original equipment manufacturers (OEMs) who build these system boards. The protocol is implemented over the industry-standard SMBusTM serial interface, enabling the configuration, control and real-time monitoring of power conversion products. The command language and data format standardization allows OEMs to easily develop and reuse firmware, which can help power system designers reduce product development time and speed time to market. For more information, visit: http://pmbus.org.

With more than 75 PMBus standard commands, users can take full operational control of the power system using one of the most popular open standard power management protocols. Users can also generate interrupt requests for the system controller by asserting the ALERT pin in response to supported PMBus faults. The LTpowerPlay graphical user interface provided by Linear Technology makes it very easy for users to obtain the operating status and set values ​​of the LTC3880/-1.

in conclusion

Digital power has established a new design environment for power supplies, which creates value in several ways. First, digital control of analog power supplies with a simple PC connection is very useful during the development phase, allowing designers to quickly get the system working. Multi-rail system designers need to easily monitor, control and adjust supply voltages, limits and sequencing. Production margin testing is easier to perform than traditional methods because the entire test can be controlled with a few standard commands over the I2C / PMBus bus.

Power system data can be sent back to the OEM regarding the operating status of the power supply, effectively opening up blind spots regarding the operating status of the DC/DC converter. If a circuit board is returned, the fault log can be read back to determine what fault occurred, the temperature of the circuit board, and when the fault occurred. This type of data can be used to quickly determine the root cause of the fault, determine if the system is operating outside of specified operating limits, or to improve the design of future products.

A properly designed digital power management system can provide the user with power consumption data, allowing intelligent energy management decisions to be made, which can be used to reduce overall power consumption. Digital power is not the answer to everyone's problems, but for multi-rail complex systems and OEMs who need to keep a record of the power system status, digital power is a very powerful tool.