Digital power management improves system performance while reducing energy costs

Today's network equipment designers face rapidly shrinking development time and tight cost constraints, but they are still expected to push performance limits and increase functionality. More and more 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 need, 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, thus avoiding time-consuming hardware changes. Software-based in-circuit testing (ICT) and board development and health verification are greatly simplified compared to traditional hardware ECN methods 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 logging, enabling rapid diagnosis of power system faults and prompt 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 point of load, on the board and in the rack, or even at the installation stage, thereby reducing both infrastructure costs and the total cost of ownership over the product's lifetime.

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 controller with EEPROM (only one channel shown)

PMBus INTERFACE：PMBus interface

TO/FROM OTHER DEVICES: To/from other devices

TO uP RESETB INPUT: To the microprocessor reset B input

WATCHDOG TIMER INTERRUPT: Watchdog timer interrupt signal

DC/DC CONVERTER：DC/DC converter

* SOME DETIALS OMITTED FOR CLARITY: * Some details have been omitted for clarity:

ONLY ONE OF FOUR CHANNELS SHOWN: Only one of four channels is shown

**LTC2974 MAY ALSO BE POWERED DRIECTLY FROM EXTERNAL 3.3V SUPPLY

** The LTC2974 can also be powered directly from an external 3.3V supply

Sequence any number of power supplies; add power supplies at will

The LTC2974 simplifies sequencing of any number of supplies. Using a time-based algorithm, users 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 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 supply of daughter card connector pins.

Figure 2: Multiple LTC2974s can be seamlessly cascaded with only two connections

SEQUENCE SUPPLIES UP IN ANY ORDER: Sequence the power supplies up in any order

INDIVIDUAL MARGINING FOR ALL SUPPLIES: All supplies can be margined individually

SEQUENCE SUPPLIES DOWN IN ANY ORDER: Sequence power supplies down in any order

0.5V/DIV: 0.5V per grid

AC-COUPLED: AC coupling

Power-on sequencing can be triggered in response to various 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 universal fault management

The bidirectional fault pin 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-values ​​of the limit thresholds and response times of the voltage and current supervisors are programmable. In addition, the input voltage, die temperature, and the temperature of the four external diodes can be monitored. The LTC2974 can be set so that if any of these quantities exceeds their over- or under-value limits, the LTC2974 responds in several ways, including immediate latch-off, anti-spike latch-off, and latch-off with retry.

An external microcontroller can also be monitored using the integrated watchdog timer. 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 once the power good signal is asserted. The LTC2974 can be configured to hold the microcontroller in reset for a predetermined length of time before reasserting the power good output if a watchdog fault occurs.

Improving Manufacturing Yields with Accurate Voltage Monitoring

As voltages drop below 1.8 V, many off-the-shelf modules have trouble meeting output voltage accuracy requirements over temperature. Absolute accuracy requirements of less than ±10 mV are now common, necessitating fine-tuning 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 significantly impact manufacturing yields. A much better way to address this problem is to accept the reality of power module inaccuracies and enable the system to self-tune 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 devices

TEMPERATURE: Temperature

Robust system due to very easy margining

The LTC2974's digital servo loop 10-bit DAC allows the user to adjust the power supply margin 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 characterize the load across all operating modes. FPGA users optimize code to minimize power, while ASIC users adjust core voltage based on throughput requirements. Accurate, real-time telemetry greatly simplifies this task.

Using the LTC2974, voltage, current and temperature status registers can be used to determine if the system is in a healthy state, while a multiplexed 16-bit ∆∑ ADC monitors input and output voltages, output current, and internal and external diode temperatures.

Due to the trend toward lower core voltages, accurately measuring load current has become a challenge, as using an accurate current sense resistor 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 dissipation, reduced circuit complexity, and cost. However, inductor resistance is highly temperature-dependent, and accurately measuring the temperature of the inductor core is difficult, which inevitably introduces current measurement errors (see Figure 4).

Figure 4: Thermal image of a DC/DC converter showing the difference between actual inductor temperature and the temperature monitoring point

The LTC2974 makes accurate DCR sensing possible 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 with inductors that have negligible DCR (see Figure 6).

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

INDUCTOR SELF-HEATING: Inductor self-heating

TIME: time

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

AVERAGE IOUT ERROR(FULL-SCALE %): Average IOUT error (full scale %)

CURRENT SET POINT: Current set point

PC-based design and troubleshooting

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 via 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 capability for debugging pre-release features and field failures in high reliability systems.

Working independently

The easy-to-use PC-based LTpowerPlay software allows users to configure the LTC2974 via a USB interface and a dongle card. The LTpowerPlay software is free and downloadable, allowing designers to configure all device parameters in an intuitive interface, eliminating much of the coding work in 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 verify the board development and operation status. When the built-in EEPROM configuration is completed, the LTC2974 can work completely autonomously without the need for custom software. In addition, the addition of a tiny connector allows LTpowerPlay software to communicate with the LTC2974 within the system, allowing field users to access telemetry, system status and fault logging data as needed.

in conclusion

The LTC2974 digital power manager provides unprecedented parameter accuracy, rich features and scalable modular architecture for high availability systems. Complex multi-rail system designs can be simplified with the LTC2974. The device uses the industry-standard PMBus interface, can be directly connected 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 the Linear Technology factory. Linear Technology can use your custom configuration to produce pre-programmed devices that are ready to install in your application.