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I'm designing a device with several ICs and they require so many different input supply voltages. 18V, 12V, 3.3V, 1.8V 1.3V.

18V will probably use around 80W out of 100W(20V, 5A) main supply budget and 3.3V~1.3V will probably use around 1A~2A.

So, I was thinking of using a couple of LDOs in parallel to relieve thermal issues for the 18V source and use a buck converter to make 20V into 5V and use LDO to supply 3.3V, 1.8V, and 1.3V.

But I'm not so sure if it's ok to directly connect buck converter and the LDO to a single 20V,5A power supply rail. My concern is that during operation, LDO and buck Vout could be unstable when either one pulls currents hard.

I'm thinking Al caps with high capacitance could help, but is this enough to maintain stability? Are there good guidelines for using linear regulators and switching converters?

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  • \$\begingroup\$ You will still dissipate 9 watts no matter how many LDO regulators you have in parallel and this means that the 80 watts budget for the 18 volt supply rises to about 89 watts. \$\endgroup\$ – Andy aka Feb 17 '20 at 13:39
  • \$\begingroup\$ @Andyaka, you are correct. The rest of the other supplies will use less than 10W, so that sums up to 100W. There was not much choice for 18V conversion because the buck converter efficiency was almost the same as using the LDOs. So I decided to go with the smaller footprints. \$\endgroup\$ – BrianH Feb 17 '20 at 14:01
  • \$\begingroup\$ While LDO regualators themselves may have a smaller footprint thant a buck converter, you will need a fairly large heatsink on the LDOs to deal with the 9 watt power dissipation, so the LDO solution may actually require more space than the buck converter. \$\endgroup\$ – Peter Bennett Feb 17 '20 at 16:33
  • \$\begingroup\$ @PeterBennett I forgot to add. Both small footprint and low BOM is required for my design. But yes heat sink will be big for the heat dissipation. \$\endgroup\$ – BrianH Feb 18 '20 at 0:22
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But I'm not so sure if it's ok to directly connect buck converter and the LDO to a single 20V,5A power supply rail.

It depends on the behavior of your supplies and of the rail, but in general that's not a problem.

What could be a problem even with just the one converter to 18V is that if that rail is coming from a switching regulator that's does something severe like going into fault mode on overcurrent, then you need to make sure you have a soft-start on all your supplies, and possibly strict current limiting. If you're really limited to 100W, then you don't have much overhead for inefficiency in your switchers. If your 20V, 5A supply can't handle transient overloads, then in turn you need to make sure it never sees them.

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To minimize the effect of switched current of the buck converter on other parallel things (your LDOs), it might be a good idea to put an EMI filter on the input of the buck. Be sure to design the filter to avoid buck controller instabilities by damping it properly.

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