Digital power system management eliminates “blind spots”

While power management is critical to the reliable operation of modern electronic systems, perhaps the last "blind spot" that remains in today's systems is the voltage regulator. There is no way for the regulator to directly configure or monitor its critical power system operating parameters. As a result, power designers have been forced to use a hodgepodge of sequencers, microcontrollers, and voltage supervisors to program basic regulator functions such as startup and safety. While digitally programmable DC/DC converters have been available for many years (most notably in VRM core supplies using VID output voltage control), the ability to monitor operating status information directly from the regulator (especially real-time current) has been lacking.

Digital power system management is eliminating this "blind spot" by enabling the setting and monitoring of a variety of power supply parameters through a computer interface. Programmable parameters include output voltage, sequencing, tracking, delays and ramps for multiple rails, overcurrent limit and overvoltage limit set points, and operating frequency. Digital power system management can also read back telemetry data and report input voltage, output voltage/current, temperature, and even faults.

Networking System designers of equipment are being forced to increase data throughput and performance of their systems, as well as add functionality. At the same time, they are under pressure to reduce overall system power consumption. The challenge for data centers is to re-arrange workflows to move jobs to underutilized servers, thereby allowing other servers to shut down and reduce overall power consumption. To meet these demands, it is critical to know the power consumption of end-user equipment. A properly designed digital power management system can provide users with power consumption data, allowing intelligent energy management decisions to be made.

Multi-Rail Board-Level Power Systems

Most embedded systems are powered from a 48V backplane. This voltage is usually stepped down to a lower intermediate bus voltage (such as 12V) to power the boards in the racks within the system. However, most sub-circuits or ICs on these boards require operation at voltages below 1 to 3.3V and currents ranging from tens of mA to hundreds of A. Therefore, point-of-load (POL) DC/DC converters are required to step down the intermediate bus voltage to the voltage required by the sub-circuit or IC. These rails usually have strict requirements for sequencing, voltage accuracy, margining, and supervision.

It is not uncommon to have as many as 20 POL voltage rails in a datacom, telecom or storage system, so system designers need a simple way to manage the output voltage, sequencing and maximum allowable current requirements of these rails. Many processors require the I/O voltage to rise before the core voltage, while some DSPs require the core voltage to rise before the I/O voltage. Power-off sequencing is also necessary. Therefore, designers need to make changes very easily to optimize system performance and need to store a specific configuration for each DC/DC converter to simplify the design work.

To protect expensive ASICs from possible overvoltage conditions, high-speed comparators must monitor the voltage value of each rail and take immediate protective action if a rail exceeds the specified safe operating limit. In a digital power system, the master can be notified of a fault through the PMBus alarm line, and the slave rail can be shut down to protect the powered device (such as an ASIC). To achieve this, reasonable accuracy and response time of tens of μs are required.

The LTC3880/-1 provides highly accurate digital power system management with high-resolution programmability and fast telemetry data readback, which can control and monitor key load point converter functions in real time. The device is a dual-output, high-efficiency, synchronous step-down DC/DC controller that provides an I2C-based PMBus interface with more than 100 instructions and built-in EEPROM. The device combines the best analog switching regulator controller and precise mixed-signal data conversion to achieve unparalleled ease of design and manageability of power systems, and is supported by the LTpowerPlay software development system with an easy-to-use graphical user interface (GUI). Figure 1 shows a typical application schematic.

Figure 1 Typical LTC3880 application schematic

The LTC3880/-1 can regulate two independent outputs or be configured as a two-phase single output. Up to 6 phases can be interleaved and paralleled to achieve accurate current sharing between multiple ICs, minimizing input and output filtering requirements for high current or multi-output applications. The built-in differential amplifier provides true remote output voltage sensing. The integrated gate driver powers all N-channel power MOSFETs over an input voltage range of 4.5 to 24V. The device can generate up to 5.5V at an output current of up to 30A per phase over the entire operating temperature range, while the output voltage accuracy is ±0.50%. Accurate timing and event-based sequencing covering multiple chips allow complex multi-rail systems to achieve power-up and power-down optimization. The LTC3880 has a built-in LDO to power the controller and gate driver, while the LTC3880-1 allows the use of an external bias voltage for maximum efficiency. Both devices are available in a thermally enhanced 6mm×6mm QFN-40 package.

Control interface for digital power system management

The PMBus command language was developed to meet the needs of large multi-rail systems. PMBus is an open standard power management protocol that uses a fully defined command language to facilitate communication with power converters, power management devices, and system host processors. In addition to a well-defined set of standard commands, PMBus-compatible devices can also use their own specialized commands to provide innovative methods for setting and monitoring POL DC/DC converters. The protocol is implemented through the industry-standard SMBus serial interface to set, control, and monitor power conversion products in real time. The standardization of command language and data formats allows for very easy development of firmware, thereby accelerating product launch.

The LTC3880/-1’s programmable control parameters include output voltage, margining, current limit, input and output supervisory limits, power-up sequencing and tracking, switching frequency, and identification and traceability data. Built-in precision data converters and EEPROM allow the collection and non-volatile storage of regulator configuration settings and telemetry variables, including input and output voltage and current, duty cycle, temperature, and fault history. Table 1 shows some of the parameters that can be set with the LTC3880/-1, the device’s high-resolution telemetry readback capability, and the resolution and accuracy of alternative solutions.

The configuration of the LTC3880/-1 is easily saved to the internal EEPROM through the device's I2C serial interface. Since the configuration is stored on chip, the controller can power up autonomously without burdening the host processor. Default settings for output voltage, switching frequency, phase and device address can be optionally configured through external resistor dividers. Multiple designs can be easily calibrated and configured in firmware to optimize a single hardware design for a range of applications.

Analog Control Loop

The LTC3880/-1 is a digitally programmable controller that implements a wide variety of functions such as controlling output voltage, current limit set point and sequencing, etc. The device also has an analog feedback control loop for optimal loop stability and transient response without the quantization effects of a digital control loop.

Figure 2 shows different ramp curves for a controller IC with an analog feedback control loop (LTC3880) and a digital feedback control loop. The analog loop has a smooth ramp, while the digital loop behaves like a step function, which can cause problems with stability, slower transient response, the need for larger output capacitors in some applications, and digital loop quantization effects that cause larger output ripple.

Figure 2 LTC3880 analog control loop and digital control loop

Figure 3. Comparison of transient response between LTC3880 analog loop and digital control loop

In addition, due to the presence of ADC, digital compensator and digital PWM, the quantization effect of the digital control loop increases the output ripple voltage, depending on the ADC resolution and loop design. In contrast, the analog control loop does not have this additional output ripple voltage.

in conclusion

One of the main benefits of digital power system management is reduced design costs and faster time to market. Complex multi-voltage rail systems can be developed efficiently using a comprehensive development environment with an intuitive graphical user interface (GUI). In addition, such systems can also use the GUI (rather than soldering assemblies) to make relevant adjustments, thus simplifying in-circuit testing (ICT) and board debugging.

Another benefit is that, with real-time telemetry data available, it is possible to predict power system failures and take preventive action. Perhaps most importantly, DC/DC converters with digital management capabilities allow designers to develop "green" power systems that can re-arrange workflows to shift work to underutilized servers, allowing other servers to shut down, so that such systems can decide when to reduce overall power consumption to meet target performance requirements. Digital power system management minimizes energy consumption at the load point, board, rack, and even installation, thereby reducing infrastructure costs and overall costs over the product's entire life cycle.