Power Supply Sequencing

January 15, 2011
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Voltage rails on electronics pcbs are proliferating as processors tend towards multi-voltage architectures and board designs incorporate multiple processors. Such proliferation brings complexity to the power switching task, because the supply rails must be powered up and down in the right sequence. Each processor’s voltage and timing requirements must be considered before adding to an overall scheme that meets the needs of all the onboard processors.

Some processors use voltage – based sequencing (Turn on Rail 2 when Rail 1 reaches its set voltage) while others use time- based sequencing (Turn on Rail 1, then turn on Rail 2 after a fixed time delay). Correct sequencing on shut-down is usually as important as on start-up, and the shut-down sequence may also use dummy loads to discharge output filter capacitors. Failure to correctly implement both sequences can cause problems including latch-up and failure to operate, and reduced MTBF for the components.

This diagram shows an example of time based sequencing. Care must be taken to ensure that the 3.3 V rail does not come up before the 2.5 V output is established:This can easily be accomplished by a simple optocoupler circuit:A more complex requirement is voltage based sequencing as shown below. In this case all three outputs have to be asserted simultaneously:

A third situation can be a time delay combined with defined rise time, as shown above.

Vicor’s power modules can be integrated into a flexible sequencing scheme because of their enable/disable control as well as wide range adjustability of the output voltage.

In more complex requirements control ICs like the LTC2926 from Linear can control the task, using an adjustable master channel to control further slave outputs.

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