[Design View / Design Solution]
Designers Must Take Care When Powering High-Speed CMOS
Lower core voltages and higher speeds add up to nasty load transients that place huge demands on power-supply circuits.
DESIGN VIEW is the summary of the complete DESIGN SOLUTION contributed article, which begins on Page 2.
The frenetic pace of advances made in semiconductor process technologies inevitably increases the speed and density of IC devices. Many of these devices, such as DSP-based communication and digital TV ICs, are being designed into systems that traditionally employed lower-performance, dedicated peripheral devices. Designers, however, need to treat the power requirements of these high-performance devices with care.
Because power circuits can be challenging and time-consuming to design, many engineers opt for off-the-shelf dc-dc converter modules. This choice reduces risk and effort, but it may cost more and come up short on performance. For this reason, an in-depth cost/benefit analysis often highlights the advantages of a discrete converter design based on a commercially available step-down (buck) converter IC.
The load response, frequency, power capability, and other specifications of these discrete designs can be more finely controlled to meet critical design criteria. By following 10 key design tips, designers can achieve excellent results in dc-dc power-converter designs for high-performance CMOS ICs. The 10 tips, discussed in detail, are:
Plan your power section first.
Give each high-power device its own converter.
Isolate power and ground sections.
Don't overestimate the regulator's load response capabilities.
Provide proper bypass capacitors, large and small.
Give careful thought to capacitance values and ESR.
Don't forget the input.
Remember Ohm's Law.
Understand that capacitors differ.
Measure the noise.
HIGHLIGHTS:
Plan Your Power Section
When designing a high-performance circuit board, consider the power section at the very beginning. Calculate the maximum power requirements needed, and then choose the regulator design type..
Each High-Power Device Gets Its Own Converter
By providing a separate power converter for each high-power IC, the converter can be placed next to the device. This will reduce peak transients and the resulting EMI.
Isolate Power And Ground
Providing a separate ground path for the digital currents lowers the ground noise (a.k.a. bounce) presented to analog circuitry.
Don't Overestimate The Regulator's Load Response Capabilities
Be wary of your power regulator's ability to accommodate extreme changes in current demands. The current peak may be within the regulator's capability, but the specific regulator may not react fast enough to load changes.
Capacitance Values, ESR
A large external capacitance with a low equivalent series resistance (ESR) is needed to minimize peak deviation. But because this combo can cause instability, the low- and high-capacitance values must be carefully selected.
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