7
\$\begingroup\$

I am doing a project in PCB design and I am fairly new on this.

The board I am creating has various components among them an Ethernet, USB controller some DDR memory and an FPGA. I have found schematics of even the same components that I use, which have the power regulators and the connections needed. But I do not want to just copy them out.

So in order to determine the power supply needed, to properly power up my system, what are the key points that I should look in the components datasheets? What I need to consider when choosing regulators?

\$\endgroup\$
9
\$\begingroup\$

You add up the current draw of all the components, and then make sure the power supply can handle it. Note that you want to add up the worst case current draw to be sure it will always work over all conditions the parts are specified for.

If this is a high volume product where reliability in extreme conditions is not that important, then you can use some fudge factor to size the power supply somewhere between the max current for all components and their typical values. The more components there are, the lower the probability that they will all draw their maximum current at once. These need to be different components for this statistical analysis to be valid. Dirty Harry summed up this method well: "Do you feel lucky? Well do you, punk?".

\$\endgroup\$
7
\$\begingroup\$

If you're using a single raw supply to supply various on-board regulators, then when choosing regulators you need to determine their input drop-out voltages, their output current capabilities, their regulation specs under minimum and maximum loads and minimum and maximum raw supply voltage variations, and their worst case power dissipation.

The raw supply, then, can be chosen so that its outputs never fall below the drop-out voltages of the various regulators on the board under low line and/or high load conditions.

Of arguably more importance, however, is the OFF-TO-ON and ON-TO-OFF sequencing required to obtain an orderly system power-up and power-down.

\$\endgroup\$
  • 3
    \$\begingroup\$ +1 for mentioning system-level concerns such as sequencing \$\endgroup\$ – Spehro Pefhany Feb 2 '15 at 20:54
  • \$\begingroup\$ @Lamaseed. My pleasure; you're welcome. \$\endgroup\$ – EM Fields Feb 3 '15 at 0:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.