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I'm designing a system that needs two converters. The Vin input voltage is approximately 14.5V. I need to lower the Vin voltage to 12V and also to 5V. I chose two LM2576-5 (5V) and LM2596-12 (12V) converters, the two converters require a 100uF Cin capacitor. I put a 100uF Cin capacitor for each converter. Looking at the schematic below, as the Vin voltage goes to the two converters, the capacitors C6 and C8 in this scheme seem to be in parallel, resulting in a total capacitance of 200uF. I wonder, can this affect the functionality of the converters? The correct would be to put only one capacitor for the two converters? What can I be doing wrong by putting the capacitors in this way? enter image description here

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  • \$\begingroup\$ It depends on rated current and Vin min \$\endgroup\$ Jul 9, 2018 at 14:12
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    \$\begingroup\$ Assuming all else is specified well, this arrangement should be fine. The input capacitors are (generally) not part of the feedback or stabilization part of the converters and are used for providing locally available charge to the converter circuit. Just be sure to put each input cap near its converter. \$\endgroup\$
    – pgvoorhees
    Jul 9, 2018 at 14:12
  • \$\begingroup\$ As an aside, your C6 voltage value is low \$\endgroup\$
    – pgvoorhees
    Jul 9, 2018 at 14:14
  • \$\begingroup\$ Assuming al the other components are what they should be, I give this a 99% chance that it will just work. Place both Caps near their respective chip and give each converter it's own trace to the VIN connector and I give it a 99.5% chance it will just work. If the input voltage will never exceed 16 V then you are allowed to make C6 a 16 V rated cap. But it is better to have some margin so use a 25 V or higher rated cap. \$\endgroup\$ Jul 9, 2018 at 14:16
  • \$\begingroup\$ Why are C6 and C8 not identical? It won't be a problem if your input isn't more than 16V, but why not just use the same capacitor for both? \$\endgroup\$
    – Hearth
    Jul 9, 2018 at 14:56

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The data sheet is very clear: -

enter image description here

So you use one per instance of each LM2576 and make sure you have headroom voltage rating i.e. make C6 and C8 both 25 volt versions to be safe.

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