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I would like to power an ESP32 in my car, so the input voltage is between 11 and 15v, and the output voltage should be 3.3v to power the Arduino board.

I thought that LDO were only for low dropout voltage. However, I saw that for example, the esp32 is powered with the usb 5v interface through a LDO ASM1117 3.3v. So the dropout voltage is around 1.7V...

Is this normal? What is the highest dropout voltage usable with LDO or linear regulator?

In my case, I think that the only way to power the esp32 is to use a buck converter, but if you have some solution with a linear regulator, I am interested for my understanding!

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The term LDO has come to be misused -- I see folks calling plain old high dropout linear regulators "LDO".

  • LDO -- a linear regulator that is capable of operating with a low dropout. But they can still work just fine with a large difference between \$V_{in}\$ and \$V_{out}\$
  • Linear regulator -- any regulator that works by burning up power; i.e., a regulator whose input current is substantially the same as its output current.
  • Switcher -- any regulator that works by switching power through an inductor (I'm purposely ignoring charge pumps here). Generally more efficient than a linear regulator when \$V_{in}\$ is substantially higher than \$V_{out}\$

You can use a linear regulator, LDO or not. But at worst case it'll burn up (13V - 3.3V) * (output current + regulator current). That means that (A), you have to supply that power, and (B) you need to pay attention to thermal issues. That regulator will be the hottest part of your board, and it's heat sink may be the biggest feature of your board. They will have problems if they get too hot, and thermal issues can get very un-trivial. Maybe you can stand that, maybe not.

You could also buy a switching regulator module -- they make them to be pin-compatible replacements for the popular linear regulators. It's not at all a bad way to go if you don't have the circuits chops for a switcher, or if you just don't want to bother.

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  • \$\begingroup\$ Thanks @TimWescott for this great explanation! Here are the data : AMS1117-3.3 : 1.5V≤ (VIN - VOUT) ≤ 12V ESP32 : arround 200mA maximum Regulator current : What is it ? P = (13V - 3.3V) * (output current + regulator current) = 9.7 * (0.2 + ??) = 2W T j,t2 = Ta + P × RthJA = 90°C + 15 × 2W = 120°C < 125°C I think it is too close to the maximum rating... \$\endgroup\$ – RomainD2 Nov 7 '18 at 21:36
  • \$\begingroup\$ Furthermore, I found this on the ASM1117 datasheet : Note 2: Line and Load regulation are guaranteed up to the maximum power dissipation of 1.2 W for SOT-223, 2.2W for TO-252 and 780mW for 8-Lead SOIC. Power dissipation is determined by the input/output differential and the output current. Guaranteed maximum power dissipation will not be available over the full input/output range. I would like to use SOT223, so the maximum dissipation mentionned is 1.2W! So I think it is better to use switching converter! \$\endgroup\$ – RomainD2 Nov 7 '18 at 21:38
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    \$\begingroup\$ @RomainD2 You should also study up a bit on connecting to car electrical systems -- they're nominally 12V, but in normal operation that means up to 14.5 (or 14.8 -- I always have to check), and there's a fairly normal event called "load dump" that can bring that voltage up (briefly) to 40-ish volts. There are regulators that are specifically designed to operate off of car electrical systems; you'd do well to find one and to pay attention to any app notes that pertain to using it. \$\endgroup\$ – TimWescott Nov 7 '18 at 21:44
  • \$\begingroup\$ I wouldn't use AMS1117 in an automotive application TPS7A1633 is a better fit there. \$\endgroup\$ – Jasen Nov 8 '18 at 9:31
  • \$\begingroup\$ @TimWescott : for example on usb phone charger which are connected to the 12vdc car, are they using specific chips or simple dcdc converter as mp2407, mp1584 or similar chips ? \$\endgroup\$ – RomainD2 Nov 8 '18 at 14:10

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