Power supply management - principles and design problems

POWER SUPPLY MANAGEMENT
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PRINCIPLES AND DESIGN
PROBLEMS
Instrumentation & measurement

INDEX
1. Intorducction
2. Basic monitoring
3. Basic sequencing
3.1. High-iout ldo regulator has excellent transient response
4. Sequencing with ICs
4.1. Power up considerations
4.2. Pulsar adc examples—absolute maximum ratings
4.3. Absolute maximum ratings
4.6.Electrical characteristics

4.5. Power supply sequencers

4.4. Recommendations
5. Integrated Power System Management
6. Centralized Monitoring and Sequencing
7. Supply Adjustment
8. Flexibility
9. CONCLUSION
10. Bibliography

1. INTRODUCTION
Power supply designers are using flexible supply monitoring, sequencing, and adjustment circuits to manage their systems. This articlediscusses why and how.
The monitoring and control of a growing number of power-supply voltage rails has been vital for safety, economy, durability, and proper operation of electronic systems for many years—especially for systems employing microprocessors. Determining whether a voltage rail is above a threshold or within an operating window—and whether that voltage is powered on or off in thecorrect sequence with respect to the other rails—is crucial to operational reliability and safety.
Many methods exist to solve various aspects of this problem. For example, a simple circuit using a precision resistive divider, comparator, and reference can be used to determine whether the voltage on a rail is above or below a certain level. In reset generators, the elements are combined with a delayelement to hold devices—such as microprocessors, application-specific ICs (ASICs), and digital signal processors (DSPs)—in reset while powering up. This level of monitoring is adequate for many applications.
Where multiple rails need to be monitored, multiple devices (or multichannel comparators and their associated circuitry) are used in parallel, but increasing opportunities call formonitoring ICs that do more than simple threshold comparison.
For example, consider a common requirement for power-supply sequencing: an FPGA (field-programmable gate-array) manufacturer may specify that the 3.3-V core voltage must be applied 20 ms before the 5-V I/O (input/output) voltage to avoid possible damage when the device is powered up. Meeting such sequencing requirements may be as crucial forreliability as keeping the device’s supply voltage and temperature within specified operating limits.
Also, the number of power rails in many applications has increased dramatically. Complex, expensive systems, such as LAN switches and cellular base stations, commonly have line cards with 10 or more voltage rails; but even cost-sensitive consumer systems, such as plasma TVs, can have as many as 15separate voltage rails, many of which may require monitoring and sequencing.

Many high-performance ICs now require multiple voltages. For example, separate core- and I/O voltages are standard for many devices. At the high end, DSPs may require up to four separate supplies per device. In many cases, numerous multisupply devices can coexist in a single system that contains FPGAs, ASICs, DSPs,microprocessors, and microcontrollers (as well as analog components).
Many devices share standard voltage levels (such as 3.3 V), while others may require device-specific voltages. In addition, a particular standard voltage level may have to be independently furnished in numerous places. For example, separate analog- and digital supplies, such as 3.3 VANALOG and 3.3 VDIGITAL, may be required.Generating the same voltage numerous times may be necessary to improve efficiency (e.g., memory rails running at hundreds of amperes) or to meet sequencing requirements (3.3 VA and 3.3 VB needed by separate devices at different times). All of these factors contribute to the proliferation of voltage sources.
Voltage monitoring and sequencing can become quite complex, especially if a system must be...