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I use MP2359 (datasheet pdf) to convert high voltage into 3.3V. The input range of voltage is from 4.5V to 24V.

At low input voltages, things seem to be OK. However when I input a voltage of 21V (MP2359 allows 24V max), it sometimes failed to work when start up, the internal MOSFET seem to be punctured, and the output voltage goes to 21V as the same as the input voltage terribly, parts at the output side burned.

Here is my circuit:

schematic diagram

The output current requirement is low, about 20mA.

The input capacitor is MLCC 50V 10uF, and the output capacitor is 100uF 6.3V MLCC, they are from Samsung in 1206 package.

1N5819 is in SOD323 (0805) package, and L1 is in 0805 package.

Here is my layout:

PCB layout, without components

PCB layout, with components

So what's going on?

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  • \$\begingroup\$ The MP2359 have an absolute maximum rating of 26 V on the input. Are you sure you are not exceeding that? It's difficult to tell which component is which in your layout. Can you have part designators instead of net names? Have you tried a faster diode? What does your oscilloscope tell you about the peak voltage at pin 6? \$\endgroup\$
    – winny
    Commented Oct 13, 2016 at 6:44
  • \$\begingroup\$ Thank you and designators are added to the picture. Input voltage is 21V, it doesn't exceeds the max. Sadly I don't have an oscilloscope \$\endgroup\$
    – user123179
    Commented Oct 13, 2016 at 6:56
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    \$\begingroup\$ An 0805 size inductor for such a DCDC converter worries me. What type is that inductor ? What is its saturation current ? I also notice that the component placement is significantly different from the datasheet's example. Especially the loop D2, L1, C22 (where is it ?? the right C20 I guess) needs to be kept as small as short as possible. As it is now the ground return path is all around the chip. \$\endgroup\$
    – Bimpelrekkie
    Commented Oct 13, 2016 at 7:05
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    \$\begingroup\$ @user123179 No oscilloscope will leave you in total darkness. That distance between D2 and L1 is way too far. You are most probably killing it with overvoltage from the stray inductance in the track from L1 to D2 at 1.4 MHz. Get. Yourself. An. Oscilloscope. \$\endgroup\$
    – winny
    Commented Oct 13, 2016 at 7:20
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    \$\begingroup\$ @damien "the output voltage goes to 21V as the same as the input voltage terribly" = most probably internal short since it did regulate at that load before going up in voltage, but I see what you mean. \$\endgroup\$
    – winny
    Commented Oct 13, 2016 at 12:42

1 Answer 1

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A Ceramic input cap on its own can be a very risky proposition, please tell me there is something of the few hundred uF with some ESR on VBUS somewhere reasonably close?

That 10uF input MLCC that probably has no ESR to speak of forms a high Q L/C network with the supply lead inductance that can cause a short term overvoltage on startup as it rings down I would not be that surprised to see 30+V on that node for a few microseconds if you connect a stiff power supply via a few feet of wire. The cure is a bog standard 100uF electrolytic across that 10u ceramic, to provide a lossy component to damp the resonance (An alternative is a big MLCC with a small value series resistor), or even a few tens of ohms or so in series with VBUS (You are only drawing a few mA, so the power will be tiny).

21V with a 24V abs max is really uncomfortable, I would have picked at least a 36V max input switcher if using a 21V supply, just to deal with startup transients. For reliability running anything above 75% of rating is not usually a good idea (sometimes you have no choice, but...).

Now your layout sucks, the datasheet will have a suggested layout, and at these speeds you really want to follow it, or at least respect the high dI/dt loops..

Feedback would be better from the VDD net and not directly from the end of L1, minor detail.

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  • \$\begingroup\$ Yep, I've recorded spikes of 9V at the USB device end for 5V USB when long-ish cables were used (5 or 6 foot). I'd hate to see what spikes look like with 21V on VBUS. Also, if OP is trying to meet USB spec, then he cannot exceed 10 uF capacitance. Additional capacitance can be gently switched in after enumeration but gets complicated. Easiest is to just design for large voltage spikes, maybe add some transient voltage protection, maybe add a snubber. \$\endgroup\$
    – Vince Patron
    Commented Jan 30, 2017 at 3:39
  • \$\begingroup\$ I didn't see any mention of USB being a consideration, but that would just add to the fun. Does USB-C really put 20 odd volts on the bus before enumeration? Still a 26V ABS MAX on a 21V bus is just asking for smoke in my view. I dont see transient protection helping here the tolerances are too tight to get to clamping voltage before the smoke comes out of that switcher. \$\endgroup\$
    – Dan Mills
    Commented Jan 30, 2017 at 10:47
  • \$\begingroup\$ OP did not mention needing to meet USB spec but I mentioned it as an FYI. And no, Type-C does not put out higher than 5V without handshaking first. But OP seems to have a unique/non-standard application and might have 21V live at the time. Also, I wonder if inductance from long cable coupled with a low ESR input cap could add stability or overshoot problems. Could simulate if this was Linear Tech. OP should put a scope on input and output to check for overshoot at startup and at load transients. \$\endgroup\$
    – Vince Patron
    Commented Jan 30, 2017 at 17:40

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