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Why is my regulator's transient response so bad?

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mck1117:
I'm having problems with the transient response of the 3.3v regulator on my board (automotive ECU, https://github.com/mck1117/proteus/).

The power supply architecture is:

Car electrical system (10-15v) -> Buck regulator (5v) -> Linear regulator (3.3v)

The only real load on the 3.3v rail is an STM32F767.  The trouble is that there's an interrupt that fires reliably at 10khz to do an ADC read and a little bit of work, so the CPU is reliably waking up then going back to sleep after a few us, at 10khz.  When transitioning from asleep to awake, the current consumption increases from perhaps 100mA to 160mA, in less than the wakeup time of 13 clock cycles (~60ns).

Here's what the transient response looks like.  AC coupled probing a test pad on the 3.3v rail.



You can see occasional other interrupts waking the CPU up, but they all look the same.



Around 70mvpp and 120khz, fourish cycles before it's damped out.

I went ahead and measured the 3.3v rail on an STM32F7 discovery board with a BGA version of the same CPU, running the same software as my custom board.  Of course the 3.3v rail wasn't perfectly flat, but it looks like what I'd expect: the regulator smoothly stepping between two voltages (slightly lower voltage under more load) with no ringing or funny business.

It seems to me like the regulator on my board is marginally stable under these conditions.

Here are the differences I can identify between the ST-designed board and mine:

* Different 3.3v regulator.  I have an Infineon TLS205, and ST used an ST LD39050.
* Slightly different 3.3v rail capacitors.  I have all ceramics, and ST used mostly ceramics but a few tantalums, but only for VDDA and VREF+ (but not the cap right next to the regulator - that one's ceramic)
* Layout.  I don't think this is it though, since there's a ~30mm square 3.3v plane underneath the regulator and STM32, with no trace to the power/ground planes longer than around 3mm for any pin, regulator or STM32.  It also isn't a local effect: measuring any point on the 3.3v net looks identical.  Besides, 120khz is much too slow for an LC resonance somewhere on the board.
Any ideas what's going on?

Does my choice of regulator suck, or did I do something wrong here?

I designed a tiny board to act as an interposer between the SOIC8 footprint on the board, and an LD39050 reg to try the different regulator on my board, so in a few days I'll give that a try.

mck1117:
Oh, and here's the relevant section of the schematic:



I've tried a range of values for C903, from a few uF up to double stacked 22uF for a total of 44uF.  There's also an additional ~8uF worth of decoupling capacitors scattered around the 3.3v plane.

T3sl4co1l:
Hmm, layout?

Tim

mck1117:

--- Quote from: blueskull on February 15, 2020, 03:39:48 am ---The datasheet specified a minimum ESR for stability. Does your circuit have one? MLCCs have very low ESR, you might want to add a bit.

--- End quote ---

The datasheet claims a minimum esr below their ability to measure at 0.06 ohm, but decreasing with increased capacitance.  My caps are certainly below that, so that could be it.  The datasheet claims that it's stable with ceramic caps, but maybe they define what I'm seeing as stable (which it technically is, I suppose).

Looking at this TI doc: http://www.ti.com/lit/an/slva115a/slva115a.pdf, it claims that fewer than 4-ish rings on a load step indicates sufficient phase margin for stability, and I'm seeing...almost exactly 4, so it's pretty borderline.


--- Quote from: blueskull on February 15, 2020, 03:39:48 am ---Also, the datasheet does NOT specify full range step response, so you might want to give it a few mA of load to keep it from entering low power mode.

--- End quote ---
And I'm not giving it one - the peripherals are still powered in sleep mode.  It's dropping perhaps 1/2 to 1/4 of the load.


--- Quote from: blueskull on February 15, 2020, 03:39:48 am ---Since you are building an ECU, I guess power consumption is no that important.

--- End quote ---

Correct.  I care more about power dissipation than power consumption, but anything within the range of a few watts is fine.  Current total for the board is only around a watt.

mck1117:

--- Quote from: T3sl4co1l on February 15, 2020, 03:53:10 am ---Hmm, layout?

Tim

--- End quote ---

Here's a snip of the full area of the 3.3v pour (internal layer, pink outline).



The 3.3v net is highlighted.  The TLS205 is at the top left, on the back layer (green), and the STM32 is the lqfp144 on the front layer (red).

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