Advertisement Feature – Gregory Manlove at Linear Technology Corporation explores the impact of a dual output DC/DC controller that clearly combines digital power system management with analogue control loop
Though power management is critical to the reliable operation of modern electronic systems, few regulators have the means for direct configuration or monitoring of key parameters. As a result, power designers who want digital control must use a combination of sequencers, microcontrollers and voltage supervisors. In response to this, Linear Technology has developed its LTC3880/-1 to solve the problem of complex power system management.
This device combines a dual output synchronous step-down DC/DC controller with a comprehensive power management feature set accessed via the I2C-based PMBus. PMBus can be used to set the output voltage, margin voltages, switching frequency, sequencing and a number of other operating parameters.
The controller integrated into this device also allows monitoring of the supplies via a 16-bit data acquisition system, which supplies digital read back of input and output voltages and currents, duty cycle and temperature, including peak values of important parameters. It also includes extensive fault logging capability along with a nonvolatile memory recorder, which stores the state of the converter’s operating conditions immediately prior to a fault.
A principal benefit of digital power system management is reduced design cost and faster time to market. The design of complex multirail systems can be simplified by using this device as it incorporates free, downloadable LTpowerPlay development software, which is a comprehensive PC-based development environment.
Power supply bring up and board debug is said to require only a few clicks of the mouse – as a result there is no need to solder in fixes. Design modifications are immediately accessible, due to the availability of real-time telemetry data, making it possible to predict power system failures and immediately implement preventive measures.
Perhaps most significantly, DC/DC converters with digital management functionality allow designers to develop green power systems that optimise energy usage while meeting system performance targets (compute speed, data rate, etc.). Optimisation can be implemented at the point of load, at the board, rack and even at installation levels, reducing infrastructure costs and the total cost of ownership over the life of the product.
To provide best performance regulation, the LTC3880 utilises a precision reference and temperature-compensated analogue current mode control loop to produce tight ±0.5 percent DC output voltage accuracy. The analogue control loop is calibrated to be independent of operating conditions, yielding cycle-by-cycle current limit, and producing a fast and accurate response to line and load transients-without any of the ADC quantisation-related errors found in products utilising digital control.
The LTC3880 features an on-chip regulator for increased integration, whereas the LTC3880-1 allows for an external bias voltage for highest efficiency. Both parts are available in a thermally enhanced 6mm by 6mm QFN-40 package.
The LTC3880/-1 is digitally programmable for numerous functions including the output voltage, current limit, and sequencing. The analogue control loop offers loop stability and transient response without the quantisation effect of a digital control loop.
Figure 2 (right) illustrates the ramp curves of an analogue feedback control loop in comparison to a digital feedback control loop. The analogue loop has a smooth ramp, whereas the digital loop has discrete steps that can result in stability problems, slower transient response, more required output capacitance in some applications and higher output ripple and jitter on the PWM control signals due to quantisation effects.
Figure 3 (below left) compares the transient response of the LTC3880’s analogue control loop to a competitor’s digital control loop. This device yields cleaner results with approximately half the output capacitance of the digital controller. It is designed so the transient response and compensation circuitry are not affected by programmed output voltage and current limit.
The LTC3880/-1 features digital programming and read back for real-time control and monitoring of critical point-of-load converter functions. Configurations are downloaded to internal EEPROM via the I2C serial interface supported by the company’s LTpowerPlay PC-based development software. This is available as a free download at www.linear.com/ltpow erplay. It works in conjunction with other controller and companion ICs from the company in order to quickly and easily configure multiple rail power systems.
Figure 4 (below right) shows the LTpowerPlay development platform with a USB to I2C/SMBus/PMBus adapter. After the configuration file is stored on-chip in nonvolatile memory, the controller powers up autonomously without burdening the host. Configuring a board is a simple task that requires zero firmware development.
PMBus functions include the ability to program specific power supply management parameters including:
- Output/margin voltages
- Current limit (temperature-compensated)
- Switching frequency
- Overvoltage and undervoltage highspeed supervisor thresholds
- Output voltage timing
- Input voltage on/off
- Responses to faults
- Fault propagation
In addition, PMBus functions allow monitoring of power supply operation including:
- Output/input voltage
- Output/input current
- Internal/external temperatures
- Status
- Peaks
A large multirail power board is normally composed of an isolated intermediate bus converter, which converts relatively high voltage from the backplane to a lower intermediate bus voltage (IBV), typically 12V, which is distributed around a PC card.
Individual point-of-load (POL) DC-DC converters step down the IBV to the required rail voltages. Figure 5 shows how a multirail system can be controlled with various companion ICs and DC/DC converters PMBus devices. The point of load DC/DC converters can be self-contained modules, monolithic devices or solutions comprising DC/DC controller ICs, associated inductors, capacitors and MOSFETs. These rails normally have strict requirements for sequencing, voltage accuracy, overcurrent and overvoltage limits, margining and supervision.
The sophistication of power management is increasing. It is not uncommon for circuit boards to have over 30 rails. Adding digital power system management circuitry requires minimal board space and external pins. The system must be easily modified by the user or a system host processor.
The LTC3880 works seamlessly with other Linear Technology PMBus supervisors, companion ICs and regulators from the company for optimal control of complex boards. These systems operate autonomously after initial configuration or communicate with the host for command, control and to report telemetry.
PMBus controllers and companion ICs make it easy to program power-up and power-down sequencing for any number of supplies. By using a time-based algorithm, users can dynamically sequence rails on and off in any order with simple programmable delays. Sequencing across multiple chips is made possible using the 1-wire SHARE_CLK bus and one or more of the bidirectional general purpose IO (GPIO) pins.
Additional PMBus chips can be added later without having to worry about system constraints such as a limited number of connector pins. Multiple addresses are supported in PMBus allowing over 100 unique devices on the same I2C bus.
Rail sequencing to the on state is typically initiated when the intermediate bus voltage exceeds a programmed threshold. Alternatively, rail on sequencing can be initiated in response to the rising edge of the RUN/CONTROL pin or by PMBus command.
The GPIO pins on the LTC3880 can be shared with fault pins from LTC PMBus companion ICs to control fault response dependencies between rails. For example the system can be configured such that a fault on one rail can initiate the shutdown of any number of rails. If the fault response is configured for ‘immediate off no retry’ and a fault occurs, the host must take action for the rails to be restarted.
Alternatively, if the fault response is set to ‘immediate off infinite retry’ and a fault occurs, the rail attempts to power up autonomously with user programmable delays in a hiccup mode. The fault response can also be set to ‘ignore,’ where the ALERT pin is pulled low in response to a fault, to alert the host of an issue, but the power supply continues to deliver power to the load.
The LTC3880/-1 combines analogue switching regulators with precision data conversion and a flexible digital interface for unsurpassed performance. Multiple LTC3880s can be used with other LTC products to create optimised multirail digital power systems. All the company’s PMBus products are supported by the LTpowerPlay software development system.
Digital control over analogue power supplies enables designers to get systems up and running quickly providing an easy way to monitor, control and adjust supply voltages, limits and sequencing. Production margin testing is easily performed using standard PMBus commands. Debug is also simplified because status is communicated over the bus.
Power system data can be sent back to the OEM providing information about the power supplies health and energy consumption. If a board is returned, the fault log can be read to determine which fault occurred, the board temperature and the time of the fault as well as historical data leading up to the fault.
This data can be used to quickly determine root cause, whether the system was operated outside of its specified operating limits, or to improve the design of future products. Power consumption data can be used to reduce overall power use in real time.
Linear Technology Corporation
