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I'm trying to build a buck circuit to power an ESP-2866 and neopixel LEDs for a project. I'm using the fixed 5 V version. The LEDs are powered from 5 V and there's a 3.3 V linear converter downstream that delivers power to the micro.

I've followed the datasheet parameters and also simulated the circuit with TI's Webench, but I can't get it to work. I get no output from the LM2596 and both the IC and the inductor get extremely hot. The parameters used are:

  • Input: 10 V nominal (12 V max, 9 V min)
  • Output: 3 A max (but this is just peak, the LEDs are off most of the time)

This is my schematic:

LM2596 circuit schematic

And this is the layout of my PCB (top and bottom):

Top layer

Bottom layer

Summarized BOM (all recommended by Webench design):

  • L1: Coilcraft MSS1210 AEC-Q200 33μH 20% 5.9A
  • C_IN1: 16V 39μF ESR 27mΩ
  • U5: LM2596SX-5.0/NOPB
  • D_REG: 200V 10A 900mV @ 10A DPAK Schottky
  • C_OUT1: 4.7μF ±10% 10V 1400mΩ @ 100kHz -55°C ~ +125°C CASE-A_1206 Tantalum
  • U4 and C8 are the 3.3V linear converter and its input filter cap.

(Side question: does it make any sense to have a filter cap after an output filter cap? And should I isolate the buck with a diode here?)

I've tried to have thick traces and keep them short. The LM2596 ICs, the caps and coils were from Mouser. I've tried replacing them in case I fried them soldering or testing, but nothing. I even tried a different LM2596, one from XLSEMI in case I got a batch of bad ones and still no dice.

I don't have any shorts (that I've been able to find). Pin 1 receives 12 V fine (12.3 V in fact, a bit high but within tollerance?), but the output on pin 4 is 0.3 V. I don't know what else to test or if it's just an error in my layout.

I'd appreciate any pointers or help.

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    \$\begingroup\$ Maybe the linear regulator or one of the caps has failed shorted. Check a spare blank PCB for shorts, then try removing the LDO regulator and replacing the caps. I agree with CFC that the caps seem a bit light for those currents, but I don't think that's your immediate problem (though it's possible that high spikes on the output has killed the LDO). \$\endgroup\$ Commented Apr 20, 2020 at 3:00
  • \$\begingroup\$ Is there any way to test if its the LDO? There was a minimum load on the system when I tested, about 0.2A, so the amount of current wasn't sufficient to burn it out, i think. \$\endgroup\$
    – Reven
    Commented Apr 20, 2020 at 5:03
  • \$\begingroup\$ Remove it from the board or try to measure if it is drawing the current from trace voltage drop or from it getting hot. \$\endgroup\$ Commented Apr 20, 2020 at 5:16
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    \$\begingroup\$ Replacing the linear regulator worked. I left the caps the same, as the simulator gave me those values and they were ok for my application. \$\endgroup\$
    – Reven
    Commented May 24, 2020 at 20:03
  • \$\begingroup\$ Can you tell the type of the 10A DPAK Schottky, provide (access/a hyperlink to) a datasheet? Is it a single diode package? \$\endgroup\$
    – greybeard
    Commented Jan 21, 2023 at 7:47

3 Answers 3

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The input and output capacitor need to be much higher.
Input = 680 μF
Output = 220 μF

At 3 A you will certainly need a heatsink. This regulator burns a lot.

Replace 1N5824 with SK56. You can check the working design with LM2576 which is almost the same, except it works at 52 kHz so it requires a bigger coil.

enter image description here

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Good start brother. There are two things wrong in the design.

1. Circuit design.

  • Use minimum 2pcs 47 μF input capacitors in parallel.
  • Use 2pcs 220 μF output capacitors in parallel.

2. PCB Design

  • The feedback pin 4 of buck converter to be connected after filtration of output capacitors. But you have connected the feedback just after the inductor.

The feedback loop got unstable, that's why it's heating up.

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Beyond the other answers, your ground path isn't good.

If you follow the ground path between your input cap and output cap:

enter image description here

Since you have multi-layer and a ground plane on the bottom, add many via to improve that.

Also, the feedback should be after filtering as Deepak stated.

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