AN2559 Datasheet, PDF (1/35 Pages) STMicroelectronics – System power supply board for digital solutions

AN2559
Application note
System power supply board for digital solutions
Introduction
This document describes a power supply reference board designed for powering digital
applications, such as CPUs, FPGAs, memories, etc. The main purpose of the board is to
illustrate the basic principles used for the design of the power supply and to give designers a
usable prototype for testing and use.
The trend in recent years in the supplying of power to MCUs, CPUs, memories, FPGAs, etc.
is to reduce the supply voltage, increase the supply current and provide different voltage
levels for different devices in one platform. A typical example of this situation is the FPGA.
The FPGA contains a core part which works at a low level voltage, the interface part placed
between the core and the output, the system part, etc. It is important to note that each
FPGA family has a slightly different voltage level and the trend is to decrease the voltage for
each new family. The lowest operating voltage currently available is 1 V, and this can be
expected to decrease to 0.9 V or 0.8 V in the near future. A similar situation exists with other
digital applications. Typically, the main CPU, memory and interfaces require different supply
voltage levels. Low operating voltages also present another challenge - transient. Digital
devices are typically sensitive to voltage level. If the voltage drops below or crosses over a
specific limit, the device is reset. This limit is typically ± 3 or ± 5%. On the other hand, digital
device consumption can change very quickly (several amps in a few hundred nanoseconds).
A power supply must be able to react very quickly with a minimum of over (or under) voltage,
especially in cases where very low output voltage is required. There is additional stress
placed on power supplies for digital applications in the industrial environment.
The industrial standard bus is 24 V, but this voltage fluctuates and the maximum input
voltage level required can reach 36 V. Additional surge protection is also a mandatory part of
power supply input for industrial applications.
The goal of the board described in this application note is to cover all of the issues outlined
above. It is intended mainly to satisfy industrial input requirements (operating voltages up to
36 V) and generate several output voltages for mid-range power applications (up to several
amps). The main output voltage level can simply be set.
September 2007
Rev 1
1/35
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