10 July 2012
Multichannel regulators enable smaller, more reliable embedded power solutions
Power management is commonplace in desktop computers and battery powered embedded systems, but is often ignored in embedded systems featuring a reliable mains power supply. However, end users and system designers are becoming conscious of the cost and undesirability of uncontrolled power usage.
Fortunately, power management is relatively easy to integrate into mains powered embedded systems and, from a system development perspective, efficient design also reduces thermal issues, leading to benefits elsewhere.
The starting point is to look at the requirements of the processor and the board architecture. Modern microprocessor and fpga based systems require increasing numbers of voltage rails to supply the core, interface, memory and precision analogue devices used in the system.
Typically, today's microprocessor based systems use discrete switching regulators and low drop out regulators (LDOs) to deliver power; however, as board area shrinks, this complicates the design task. Combining multiple switching regulators and LDOs into one package enables small, flexible, efficient power management solutions.
Integration of multiple devices into one package brings four key advantages: size reduction; greater ease of use; higher reliability; and lower noise.
Physical dimensions matter
While solution size is critical for portable designs and for embedded applications, where space is at a premium, it is equally important for mains powered solutions. Shrinking the power supply cuts manufacturing costs by reducing the size and number of components required. It lessens the impact on the environment and cuts transportation costs.
The recent innovation of integrating multiple switching buck regulators, LDOs, supervisory and watchdog functionality in to a single chip solution enables designers to significantly decrease the pcb area of a multirail power supply. A good example of this is Analog Devices' ADP5034: a dual 1.2A buck regulator with two 300mA LDOs in a 24LFCSP package (see fig 1).
Higher levels of integration allow the use of fewer, smaller external components. Integrated switching regulators operate at 3MHz, which allows very small chip inductors to be used and, when both switching regulators are enabled and operate in PWM mode, they are configured to run out of phase. This also reduces the size and cost of the input capacitors required.
Comparing the layout of a power solution based on the single chip multioutput regulator with the discrete approach in fig 2 shows the discrete solution requires 22 components to be placed in 97mm2 of board space, whereas the integrated solution has 19 components placed in an area of 72mm2.
As design cycles continue to shrink, device manufacturers are responding with power solutions that are not only easy to design, but also simple to modify in the future. This enables designs to be completed without the intricate knowledge and experience usually associated with complex power supplies; it enables design teams to accelerate the development process and meet tighter product release schedules.
Integrated regulators with dedicated Enable pins allow the power supply designer to enable or disable each regulators in hardware without software overhead, allowing power supply rails to be sequenced easily. The ability to set outputs of individual regulators with an external resistor divider is another innovation, allowing designers to change output voltages during prototyping. New designs requiring a different combination of output voltages are therefore easier to implement.
Multi regulators like the micro PMU shown in fig 2 have internal compensation designed for a range of I/O voltages and output capacitance. In addition, soft start and protection circuits are integrated. All these features reduce design and troubleshooting time.
Device manufacturers are simplifying board layout and placement with pinouts configured to allow passive components to be placed as close as possible to each regulator. Engineers with little or no knowledge in power circuit design need no longer fear using sophisticated multioutput regulators; all that is needed is to follow the manufacturer's board layout and component selection guidelines.
Higher reliability systems
Designs using a multiregulator improve early failure reliability compared with discrete solutions, because there are fewer components to be placed and inspected on the pcb. There are also fewer devices and circuit connections that may fail later in the product lifecycle. Furthermore, by integrating supervisory and watchdog timers along with multiple regulators into single chip solutions, reliability and longevity are improved at the system level.
Some multioutput regulators take enhanced reliability a step further, by integrating high accuracy power on reset circuits that monitor the I/O voltage rails. In a typical microprocessor based system, a power on reset is used to ensure the core voltage rail is at the correct level before taking the processor out of reset. Monitoring these rails helps to ensure more reliable end products.
As core voltage rails on new microprocessors and fpgas continue to fall, the need for high accuracy power on reset has become more important. For example, the ADP5041 provides an externally resistor programmable power on reset with ±1.5% accuracy over the full temperature range, enabling the low voltage core rail of the newest generations of microprocessors, asics and fpgas to be monitored precisely. This also has an integrated watchdog timer, allowing the microprocessor code execution activity to be monitored and guaranteeing safe, reliable processor operation.
To increase reliability and uptime of remote systems, such as electric meters, a second watchdog is integrated into the regulator. This allows a remote system to be automatically 'power cycled' if the system does not operate or respond correctly.
Low noise solutions
Electromagnetic noise continues to bedevil the designer of microprocessor based embedded systems and the problem is worsening with design complexity. Power supplies are frequently the source of this undesired noise.
The impact of electromagnetic noise can be reduced through care in the positioning of device pin outs. Enabling external passive components to be placed as close as possible to each regulator minimises the effect of board parasitics and generated noise.
Another source of power supply noise is switchers operating in burst mode. Forcing switching regulators to operate in constant PWM mode eliminates this problem. Providing a dedicated MODE pin on the multiregulator allows it to be controlled by a microprocessor I/O port, useful when the supplied circuit is sensitive to wideband noise.
To further reduce noise, new integrated power devices have been optimised by providing a low input voltage range on the integrated LDOs. This allows them to deliver high efficiency low noise outputs by combining one of the buck regulators with an LDO. For example, LDOs integrated into Analog Devices' µPMUs have inputs ranging from 1.7 to 5.5V. The buck regulators can be used as preregulators to give a high efficiency drop from a 5V input to 1.8V output on the buck regulator, which is then applied to the LDO's input to provide a low noise 1.2V output for powering sensitive analogue circuitry.
Integrated LDOs have a high power supply rejection ratio – even with a low Vin-Vout headroom – and low inherent noise. In addition, crosstalk between regulators has been minimised. All these characteristics are important when supplying noise sensitive circuits.
Meanwhile, the ADP5034 employs phase shift technology; the integrated step down regulators are operated 180° out of phase, reducing the need for input filtering and allowing smaller input capacitors.
David Pearson, technical director, and John Bowman, semiconductors marketing director, are with Anglia Components.
David Pearson and John Bowman
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