AMS117 with 13.8VDC Supply

[@rt]

Hi Guys
I have a project with dual supply (5 & 3.3V) provided by a couple of AMS117 devices.
Max supply voltages for the devices is 15V and 12V respectively.
I’ve seen a circuit where one AMS device was supplied with the output of another
in order to meet specification, but it seems like a weak idea for efficiency.

Would a few power diodes in series with the 3.3V regulator be a better idea?
Or something else?

Each supply powers an mcu, and there’s a few 5V stock logic chips.
I’ve never measured the current draw of such a circuit, but I don’t imagine it could
approach anywhere near 1A.

[PCB.Wiz]

I’ve seen a circuit where one AMS device was supplied with the output of another
in order to meet specification, but it seems like a weak idea for efficiency.

A problem there is short circuit can place the whole supply across one device.

Most AMS1117 parts I can find are 15V and most clones are 18V, so 13.8V regulated should be just fine.

[artag]

If you're dropping (13.8 - 3.3) across a device and taking 1A, it might happily tolerate the input voltage but it will struggle to dissipate the 10W of heat.

[Kim Christensen]

Max supply voltages for the devices is 15V and 12V respectively.
I’ve seen a circuit where one AMS device was supplied with the output of another
in order to meet specification, but it seems like a weak idea for efficiency.

The efficiency isn't going to be worse if you daisy chain the regulators. The same amount of power will be wasted if you didn't have to daisy chain. It'll just be split differently between the two regulators but the total power wasted as heat will be the same.

[mikerj]

The general advice when you have so much headroom is to not use LDO regulators.  An LM317 will work nicely with no concerns of maximum voltage and no fussiness with capacitor ESR.

[@rt]

Or I have considered mounting USB power ports on projects, which is basically handing the problem off to China
I was soldering last night under a USB powered magnifier ring lamp, using a car cig lighter adapter for USB power, which reminded me how cool these switching  converters can run.

[Doctorandus_P]

First, AMS1117 has three ones in it's number (you forgot one both in the subject title and in your text). Getting that right helps with online searches for datasheets and such.

Putting linear voltage regulators in series is "not unusual". These days, mixed logic with 5V and 3V3 chips is quite common, and there are plenty of voltage regulators for which the maximum input voltage is 6V.

Concerning "efficiency". Total heat to be dissipated with linear regulators = Voltage difference times current (which is generic formula for power) and the order does not matter.
When your input voltage is that high, consider using an SMPS if you want efficiency.