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If I am laying out two switch mode power supplies right next to one another from the same supply(24V), one is switching to 5V and the other to 12V then do I need both of the input capacitors? Thanks

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  • \$\begingroup\$ Maybe. The recommended caps probably have some engineering margin built in, so you might be able to get by with less, but each set is specified with only its own load in mind. Add another one, and it may or may not be enough for both. Also, is there a maximum capacitance for the 24V supply? Sometimes there is, sometimes not. \$\endgroup\$ – AaronD Dec 12 '14 at 21:53
  • \$\begingroup\$ Calculate input currents of both regulators, based on the ripple current required you select the Input capacitors. Generally these input capacitors should be close to their regulators. Because both regulators are different, these input capacitor places are also will be different. Find Cin required for regulator and put that capacitor near to that regulator. Even though in schematic they may look like extra. But when connected in layout, there cap's are separate by copper(fnite amt of inductnace).Calc Cin for different sources and place near to sources// \$\endgroup\$ – user19579 Mar 13 '15 at 12:55
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For a buck converter you want grounded pins of input, output caps and diode to be placed close together (among other things). This will be hard to do if two converters share the same input cap. More information about proper DC-DC converter layout can be found in Linear Tech. appnote 136:

http://cds.linear.com/docs/en/application-note/an136f.pdf

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Just add the input capacitances you would have used separately and then round up to next standard value bulk capacitor.

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The use of a common input cap is generaly better because there can be some ripple cancellation if your convertors are interleaved or to put it another way polyphased NOW even if you are just using separate chips out of the book there is still some advantage because of vectorial addition of ripple currents Even when I have done this using Variable frequency schemes to reduce switching losses and used caps rated for each job but paralleled them I have noted a reduction in ripple volts on the DC bus SO its a good idea for EMC reasons and it makes even more sense when replacing aluminium electros with Multilayer SMD ceramic caps

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