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