Today’s network equipment designers are under pressure to rapidly reduce development time and cost constraints, but they are still expected to push performance constraints and add functionality. More and more network system functions require the addition of ASICs and processors, each requiring 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 efficiently debugged through PC-based software tools, avoiding time-consuming hardware changes. Software-based in-circuit testing (ICT), as well as board development and health verification, is greatly simplified compared to traditional hardware ECN methods because firmware changes can be made on the 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 meaningfully, a DC/DC converter with digital management allows designers to develop “green” power systems that meet system performance goals (computing speed, data transfer rate, etc.) Can optimize energy utilization. Optimization can take place at the point of load, on the board and rack, and even during the installation phase, reducing both infrastructure costs and the total cost of ownership of the product over its lifetime.
This article discusses how performance, reliability, and energy efficiency can be improved by using the LTC2974 quad-channel digital power management IC in network switches and routers, base stations and servers, and industrial and medical equipment.
picture1:haveEEPROM ofFourChannel Power Controller(Only one channel is shown)
PMBus INTERFACE: PMBus interface
TO/FROM OTHER DEVICES: To/From other devices
TO uP RESETB INPUT: Reset B input to microprocessor
WATCHDOG TIMER INTERRUPT: Watchdog timer interrupt signal
DC/DC CONVERTER: DC/DC converter
* SOME DETIALS OMITTED FOR CLARITY: * Some details omitted for clarity
ONLY ONE OF FOUR CHANNELS SHOWN: Only 1 of the 4 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, the user can dynamically sequence power 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 supply of daughter card connector pins.
picture2: With just two connections, you can seamlessly cascade multipleLTC2974
SEQUENCE SUPPLIES UP IN ANY ORDER: Sequence power-ups in any order
INDIVIDUAL MARGINING FOR ALL SUPPLIES: All supplies can be individually margined
SEQUENCE SUPPLIES DOWN IN ANY ORDER: Sequence power downs in any order
0.5V/DIV: 0.5V per division
AC-COUPLED: AC coupling
Power-up sequencing can be triggered in response to various conditions. For example, the LTC2974 can automatically sequence when the intermediate bus voltage of a downstream DC/DC POL converter exceeds a certain turn-on voltage. Alternatively, the turn-on sequence can be initiated by a rising or falling edge of the control pin input. The device also provides immediate disconnect or disconnect sequencing in response to fault conditions. Sorting can also be done by simply I2C command starts. The LTC2974 supports any combination of these conditions.
Robust systems require universal fault management
Bidirectional fault pins can be used to correlate fault responses between channels. For example, if a short circuit occurs, the turn-on sequence of one or more channels may be terminated. The limit thresholds and response time overshoot and undershoot values of the voltage and current supervisors are programmable. In addition, input voltage, die temperature and temperature of 4 external diodes can be monitored. The LTC2974 can be programmed so that if any of these quantities exceed their over or under limits, the LTC2974 responds in several ways, including immediate lockout, anti-spike lockout, and Lockout of the retry function.
An integrated watchdog timer can also be used to monitor an external microcontroller. There are two timeout intervals available: first watchdog interval and follow-up interval. This makes it possible to specify a longer timeout interval for the microcontroller after the power good signal is determined. The LTC2974 can be configured to, in the event of a watchdog fault, put the microcontroller in reset for a predetermined length of time before reasserting the power good output.
Improve Manufacturing Yield with Accurate Voltage Monitoring
As voltages drop below 1.8V, many off-the-shelf modules have trouble meeting output voltage accuracy requirements over temperature. Absolute accuracy requirements below ±10mV are now common, necessitating fine-tuning of the output voltage during manufacturing, a time-consuming process.
Original equipment manufacturers (OEMs) must allow test margins to ensure reliable systems are delivered in the face of drifting rail voltages, which can dramatically impact manufacturing yields. A much better solution to this problem is to accept the reality of inaccurate power modules and allow the system to fine-tune 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 yield, the digital servo loop circumvents module accuracy limitations, making it easier to power power modules.
picture3:LTC2974 Provides excellent voltage servo accuracy over temperature
THREE TYPICAL PARTS: 3 TYPICAL PARTS
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 graphics. Margining is done with a single command via I2The C interface is controlled, and the output of the margining DAC is connected to the feedback node, or the input of the DC/DC converter trimmed 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.
preciseandtemperature compensatedDCR Load current monitoring
In order to achieve the desired power savings, it is necessary to summarize the load characteristics for all operating modes. FPGA users optimize code to minimize power, while ASIC users adjust core voltages based on throughput needs. Accurate real-time telemetry greatly simplifies this task.
Using the LTC2974, the voltage, current, and temperature status registers determine whether the system is in a healthy state, while a multiplexed 16-bit ΔΣ ADC monitors input and output voltages, output currents, and internal and external diode temperatures.
Due to the trend of lower and lower core voltages, accurate measurement of load current has become a challenge because the use of accurate current sense resistors can lead to unacceptable power losses. One option is to use the Inductor‘s DC resistance (DCR) as a current shunt component. There are several advantages to doing so, including zero additional power dissipation, lower circuit complexity and cost. However, inductor resistance is strongly temperature-dependent, and it is difficult to accurately measure the temperature of the inductor core, which inevitably introduces current measurement errors (see Figure 4).
picture4: oneDC/DC Thermal image of converter showing difference between actual inductor temperature and temperature monitoring point
The LTC2974 enables accurate DCR detection 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 is compatible with the LTC2974’s low noise 16-bit ∆∑ Combined with an ADC, an accurate measurement of load current can be achieved using an inductor with minimal DCR (see Figure 6).
picture2:LTC2974 Compensating inductor self-heating with thermal resistance and delay parameters
Inductor SELF-HEATING: Inductor self-heating
picture6: over temperature and output current range, for aDC/DC converterLTC2974 Total current measurement error
AVERAGE IOUT ERROR(FULL-SCALE %): Average IOUT Error (% of full scale)
CURRENT SET POINT: Current set point
based onPC 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 data from the last 500ms of ADC telemetry are provided. If a channel is disabled in response to a fault, the LTC2974’s data record can be stored in a protected EEPROM. This 255-byte block of data remains in non-volatile memory until the I2C command to clear.
picture7:LTpowerPlay The software allows designers to connect thePC Plug into the system so that the power management system can be fully configured and controlled without writing a single line of code.
Figure 7 shows the data record content as seen in the LTC2974 interface of LTpowerPlay. In this way, the LTC2974 provides a complete snapshot of the state of the power system before a critical fault occurs, making it possible to isolate the source of the fault as soon as it occurs. This is an invaluable capability for debugging pre-release features and field failures of high reliability systems.
The easy-to-use PC-based LTpowerPlay software allows the user to configure the LTC2974 via the USB interface and a dongle card. LTpowerPlay software is freely available and downloadable, allowing designers to configure all device parameters in an intuitive interface, eliminating extensive coding during development and accelerating time-to-market.
Once the device configuration is finalized, the designer can save the parameters to a file and upload it to Linear Technology’s factory. Linear Technology can use this file to pre-configure devices, allowing customers to most easily verify board development and health. When the built-in EEPROM is configured, the LTC2974 can operate completely autonomously without the need for custom software. In addition, the addition of a tiny connector allows LTpowerPlay software to communicate in-system with the LTC2974, allowing field users to access telemetry, system status and fault log data as needed.
The LTC2974 digital power manager provides unprecedented parametric accuracy, rich features and a scalable modular architecture for high availability systems. Complex multi-rail system designs can be simplified with the LTC2974. The device utilizes an industry-standard PMBus interface for direct connection to the powerful, PC-based and freely available LTpowerPlay control software, and includes an integrated EEPROM for complete customization. Customers can use the LTpowerPlay design tool to design applications and easily upload the configuration to Linear Technology’s factory. Linear Technology can produce ready-to-install pre-programmed devices in your application with your custom configuration.