Introduction to high performance and low cost digital power management

Today's network equipment designers face the pressure of rapidly shortening development time and strict cost constraints, but they are still expected to break through performance limitations and increase functionality. Increasing network system functions require the addition of ASICs and processors, each of which requires several voltage rails, resulting in line cards with dozens of rail voltages. The challenge with so many voltage rails is to optimize hardware utilization to minimize overall power consumption.

To meet this demand, digital power management is rapidly emerging as a key component of complex high-reliability applications. Digital power management allows complex multi-rail systems to be debugged efficiently through PC-based software tools, avoiding time-consuming hardware changes. Compared with traditional hardware ECN methods, software-based in-circuit testing (ICT) and board development and health verification are greatly simplified because firmware changes can be completed on a PC without touching the board. Digital power management provides designers with real-time telemetry data and fault records, enabling rapid diagnosis of power system faults and rapid corrective action.

Perhaps most significantly, DC/DC converters with digital management capabilities allow designers to develop "green" power systems that optimize energy utilization while meeting system performance goals (computing speed, data transfer rate, etc.). Optimization can be performed at the load point, circuit board, and rack, or even at the installation stage, thereby reducing both infrastructure costs and the total cost of ownership over the product's entire life cycle.

This article explores how to improve performance, reliability, and energy efficiency by using the LTC2974 quad-channel digital power management IC in network switches and routers, base stations and servers, and industrial and medical equipment.

Figure 1: Quad-Channel Power Supply Controller with EEPROM (Only One Channel Shown)

PMBus INTERFACE TO/FROM OTHER DEVICES TO uP RESETB INPUT WATCHDOG TIMER INTERRUPT DC/DC CONVERTER * SOME DETIALS OMITTED FOR CLARITY ONLY ONE OF FOUR CHANNELS SHOWN ** LTC2974 MAY ALSO BE POWERED DRIECTLY FROM EXTERNAL 3.3V SUPPLY Sequencing Any Number of Power Supplies; Add Supplies at Will The LTC2974 simplifies sequencing any number of power supplies. Using a time-based algorithm, the user can dynamically sequence the power supplies on and off in any order. Sequencing across multiple LTC2974s is also possible using a single-wire shared clock bus and one or more bidirectional fault pins (see Figure 2). This approach greatly simplifies system design because the channels can be sequenced in any order, regardless of which LTC2974 provides control. Additional LTC2974s can be added at any time without worrying about system constraints, such as limited pin availability on daughter card connectors.

Figure 2: Multiple LTC2974s can be seamlessly cascaded with just two connections SEQUENCE SUPPLIES UP IN ANY ORDER: Sequence supplies up in any orderINDIVIDUAL MARGINING FOR ALL SUPPLIES: All supplies can be margined individuallySEQUENCE SUPPLIES DOWN IN ANY ORDER: Sequence supplies down in any order0.5V/DIV: 0.5V per divisionAC-COUPLED: AC-coupledPower-up sequencing can be triggered in response to a variety of conditions. For example, the LTC2974 can automatically sequence when the intermediate bus voltage of the downstream DC/DC POL converter exceeds a specific turn-on voltage. Alternatively, turn-on sequencing can be initiated by a rising or falling edge of the control pin input. The device also provides immediate turn-off or turn-off sequencing in response to fault conditions. Sequencing can also be initiated by a simple I2C command. The LTC2974 supports any combination of these conditions.

Rugged systems require common fault managementBidirectional fault pins can be used to establish correlation of fault responses between channels. For example, if a short circuit occurs, the turn-on sequencing of one or more channels can be terminated. The over- and under-limit values ​​of the limit thresholds and response times of the voltage and current supervisors are programmable. In addition, the input voltage, chip temperature, and the temperature of four external diodes can be monitored. The LTC2974 can be set to respond in several ways, including immediate latching, anti-spike latching, and latching with retry, if any of these quantities exceeds their over- or under-limit limits. An

integrated watchdog timer can also be used to monitor an external microcontroller. Two timeout intervals are available: the first watchdog interval and the follow-up interval. This makes it possible to specify a longer timeout interval for the microcontroller after a power good signal is determined. The LTC2974 can be configured to put the microcontroller in a reset state for a predetermined length of time if a watchdog fault occurs, and then re-determine the power good output.

As voltages drop below 1.8V, many off-the-shelf modules have trouble meeting output voltage accuracy requirements over temperature. Absolute accuracy requirements of less than ±10mV are now common, necessitating trimming of the output voltage during manufacturing, a time-consuming process.

Original equipment manufacturers (OEMs) must allow margin for testing to ensure reliable systems are delivered in the face of drifting rail voltages, which can lead to significant manufacturing yield losses. A much better way to address this problem is to accept the reality that power modules are inaccurate and enable the system to trim itself in the field. The LTC2974’s digital servo loop externally trims the module’s output voltage to better than ±0.25% accuracy over temperature (see Figure 3), minimizing rail voltage drift. In addition to improving manufacturing yields, the digital servo loop circumvents module accuracy limitations, making it easier to power the power module.

Figure 3: The LTC2974 provides excellent voltage servo accuracy over temperature ERROR: Error THREE TYPICAL PARTS: 3 typical parts TEMPERATURE: Temperature-robust systems result from very easy margining The LTC2974's digital servo loop 10-bit DAC allows the user to margin the supply over a wide range while maintaining high resolution for applications such as Shmoo plotting. Margining is controlled through the I2C interface with a single command, and the output of the margining DAC is connected to the feedback node or the input of the DC/DC converter through a resistor. The value of this resistor sets a hardware limit on the allowable output voltage margining range, which is an important safety measure for power supplies under software control.

Accurate and temperature-compensated DCR load current monitoring To achieve the desired power savings, it is necessary to summarize the load characteristics in all operating modes. FPGA users optimize code to minimize power, while ASIC users adjust the core voltage based on throughput requirements. Accurate and real-time telemetry greatly simplifies this task.

Using the LTC2974, it is possible to determine whether the system is in a normal state based on the voltage, current and temperature status registers, while the multiplexed 16-bit ?∑ADC monitors the input and output voltages, output current, and internal and external diode temperatures.

With the trend toward lower core voltages, accurately measuring load current has become a challenge because using precise current sense resistors can result in unacceptable power losses. One option is to use the DC resistance (DCR) of the inductor as a current shunt component. This has several advantages, including zero additional power consumption, lower circuit complexity and cost. However, the inductor resistance is highly dependent on temperature, and it is difficult to accurately measure the temperature of the inductor core, which inevitably introduces current measurement errors (see Figure 4).

Figure 4: Thermal image of a DC/DC converter shows the difference between actual inductor temperature and temperature monitoring point The LTC2974 enables accurate DCR sensing with a patent-pending temperature compensation algorithm that compensates for the rate of temperature change from the sense diode to the inductor core, as well as the time difference that occurs between changes in inductor current and temperature (see Figure 5). This feature, combined with the LTC2974's low noise 16-bit ?∑ ADC, enables accurate load current measurement using inductors with negligible DCR (see Figure 6).

Figure 5: LTC2974 uses thermal resistance and delay parameters to compensate for inductor self-heating INDUCTOR SELF-HEATING: TIME:

Figure 6: LTC2974 total current measurement error for a DC/DC converter over temperature and output current range

PC-Based Design and Fault Diagnostics When used with LTpowerPlay? software, the LTC2974's fault and alarm registers allow designers (and field users) to determine the status of the power infrastructure at a glance (see Figure 7). In the data log, status information, available time, and the last 500ms of ADC telemetry data are provided. If a channel is disabled in response to a fault, the LTC2974's data log can be stored in protected EEPROM. This 255-byte block of data remains in nonvolatile memory until cleared with an I2C command.

Figure 7: LTpowerPlay software allows designers to plug a PC into the system through a tiny connector, allowing the power management system to be fully configured and controlled without writing a single line of code.

Figure 7 shows the data log as seen in the LTC2974 interface in LTpowerPlay. In this way, the LTC2974 provides a complete snapshot of the power system status before a critical fault occurs, making it possible to isolate the root cause of the fault as soon as it occurs. This is an invaluable feature for debugging pre-release features and field failures in high-reliability systems.

Standalone Operation The easy-to-use PC-based LTpowerPlay software allows the user to configure the LTC2974 via a USB interface and a dongle card. The LTpowerPlay software is available free of charge and can be downloaded, allowing designers to configure all device parameters in an intuitive interface, eliminating a lot of coding work during the development process and speeding up time to market.

Once the device configuration is finalized, the designer can save the parameters to a file and upload it to the Linear Technology factory. Linear Technology can use this file to pre-set the device, allowing customers to most conveniently develop and verify the board operation. Once the internal EEPROM is configured, the LTC2974 can operate completely autonomously without the need for custom software. Additionally, the addition of a tiny connector allows the LTpowerPlay software to communicate with the LTC2974 within the system, allowing field users to access telemetry, system status, and fault logging data as needed.

Conclusion The LTC2974 digital power manager provides unprecedented parameter accuracy, rich functionality, and a scalable modular architecture for high-availability systems. Complex multi-rail system designs can be simplified with the LTC2974. The device uses an industry-standard PMBus interface that connects directly to the powerful, PC-based, and free LTpowerPlay control software and includes an integrated EEPROM for complete customization. Customers can design applications using the LTpowerPlay design tool and easily upload the configuration to Linear Technology's factory. Linear Technology can produce pre-programmed devices that are ready to install in your application using your custom configuration.