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I am an engineering student and have just finished designing a PCB containing an STM32F405 MCU, an LSM9DS1 Accelerometer/Gyroscope/Magnetometer and 7 connections for potentiometer reading. My power supply is 12V/~0.02A and to power these components i have used an LM340 (to convert to 5V needed for potentiometers) and a TLE42744DV33 (to convert to 3.3V for the rest of the devices). The LM works perfectly fine outputting 5.020V but the TLE is where I have a problem. It outputs ~2.4V fluctuating between 2.2V and 2.5V. When I look to my power supply I can see 12V but the current jumps to ~0.55A. Also after a while, the regulator begins to smoke. So far the problem is observed with two brand new components and everything is hand-soldered. Also when checking 3V3 and GND in left electrolytic capacitor for short-circuit with the multimeter there was a single beep and not a constant one (which would indicate that those two networks are connected somewhere).

Schematics found below: C6->100nF,C3->10uF,C4->27uF enter image description here

What could possibly be the problem ?

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  • \$\begingroup\$ Post your schematic (and part values) as well as your layout. \$\endgroup\$ – JRE Mar 9 at 11:28
  • \$\begingroup\$ I have updated the post \$\endgroup\$ – Christos Xygkos Mar 9 at 11:34
  • \$\begingroup\$ I've updated my answer. :) \$\endgroup\$ – JRE Mar 9 at 11:48
  • \$\begingroup\$ Thank you very much for your quick reply and suggestion but in other boards i have designed and tested(but one), this problem does not exist. The problem only occurs with this one and another one. The only common thing between these two boards is the MCU. Every other PCB(that functions properly) uses either an STM32F103 or an Atmega 328p. \$\endgroup\$ – Christos Xygkos Mar 9 at 11:56
  • \$\begingroup\$ Yes. As I said, it depends on load. If the one odd MCU draws significantly more (or less) current you could have problems. \$\endgroup\$ – JRE Mar 9 at 11:58
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Going by the description of the failure and the datasheet, I'm going to guess that the output of the regulator is oscillating.

The datasheet specifies particular characteristics of the output capacitor, and mentions that they are required for stability - that is, to prevent oscillation.

Requirements:

  1. Minimum capacitance 22uF.

  2. The equivalent series resistance (ESR) of the capacitor. The chart says 3 ohms, but there's a diagram showing the relationship between load and the ESR needed for stability. You should have no more than 3 ohms, but if your circuit draws more (or less) than around 150mA you can get away with more.

You have more than enough capacitance, so I expect the problem lies in the ESR.

You mention supplying 12V at 0.02A to the whole circuit. That implies that the 3.3V load is very small, in which case you could get away with a capacitor with a higher ESR

This diagram from the datasheet shows the relationship between load current and required ESR of the output capacitor:

enter image description here

At around 150mA of load current, you may have at most 3 ohms of ESR. If your load is very low or very high, you may get away with up to 10 ohms of ESR.

If your regulator works correctly with one load, but burns out with another, then I'd compare the stability curve and the load currents for the two loads with the ESR of the output capacitor.

You can increase the ESR of your output capacitor by adding a small resistor in series, but the only way to reduce ESR is to use a different type of capacitor.

You mention electrolytics, and from the footprint shape I'd assume you are using aluminum electrolytic capacitors. You may need to switch to special low ESR aluminum capacitors or ceramic capacitors.

If you go with ceramic, remember to consider derating - you will need to use a higher rated capacitor (more than 22uF) because ceramic capacitors lose capacitance when voltage is applied. Whether it is a problem or not depends on the rated voltage of the capacitor. A 50V capacitor won't lose much capacitance at 3.3V. A 5V capacitor may lose a significant portion of its capacitance at 3.3V.


The description of the problem sounds very much like the regulator is oscillating. They tend to get hot when they oscillate, and your multimeter can't really follow the oscillating voltage and so it displays a varying voltage. They also draw a lot of current when oscillating.

Oscillate, draw a lot of current, get hot, burn out.

If you have an oscilloscope (and a spare regulator or two,) you could check the output of the regulator and see it oscillating. By the time you get done looking, though, the regulator will probably be toast.

I have killed my share of regulators by neglecting the ESR requirements. That's why I think you are experiencing this particular problem - I've had it happen, and it matches your description.

Thing is, when I encounter a misbehaving circuit, I reach for the scope first rather than the meter. I saw it oscillating before I knew the output voltage was wrong.

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