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ADP2302 self-destructs


I would appreciate some assistance in debugging my ADP2302 design.

I am using the fixed 5V version of the device to provide a switching power supply that will accept between a 9 and 20 VDC input and provide 5 VDC to my board at up to 2A.  However, in my initial testing phase my load is somewhere in the 50mA region.

The problem is that the ADP2302 doesn't seem to be switching at all. The output voltage seems to track the input voltage. As I increase the input voltage from zero to 5V, the output voltage tracks it as expected, and the current consumed from the supply also increases. However, above 5V in, I would expect to see the SW output start switching, and the current consumed from the supply start to drop.  Instead, it contines to rise. If I increase the input voltage to more than about 9V, the ADP2302 starts to overheat, and self-destructs.

I have copied the reference design and layout from the datasheet pretty faithfully, with a few substitutions for components I already had in stock.

As part of the debug I tried putting a scope probe on the SW output and then connecting Vin to an already-running 7V power source (any higher and I risk damaging things).  The waveform looked nothing like the one in the datasheet, as there was no switching.  Infortunately my scope does not have the ability to save waveforms, so I have hand-drawn it and attached to this post.  The time/div setting was the same as the diagrams in the datasheet, but instead of seeing the SW net switching on and off as the output voltage ramps up, there is a smooth curve with an initial boost up to 7V before falling back and then rising again up to the static 7V.

Obviously the output voltage should not be as high as 7V.

I will attach a screen capture of the layout of the PSU section of my board.  It has been rotated by 180 degrees so that it matches the one in the datasheet.

The bottom copper layer is blue, and the top copper layer is red.  The bottom layer power plane is GND, and I have added labels (black text in white boxes) to label the power planes on the top layer. The big green box over the centre of the ADP2302 is a solder mask keepout to create a nice large heatsink pad.  There are also two vertical rows of vias under that pad to conduct heat to the GND plane on the underside.

I will also attach a schematic of the circuit.  In addition to the components in the datasheet, I have also added RFI filters on the input and output.

The VCC5VPSU on the right of the schematic is connected through an ideal diode to the main 5V power plane of the board (to allow the board to be powered by another power source without competition if necessary)

The components I selected are as follows:

C2: Samsung CL32A106KA9LNNE Ceramic X5R 25V 10uF

C3-5: Samsung CL32A226KAJNNNE Ceramic X5R 25V 22uF

L1: NIC NPIS27H 10uH 0.0216DCR 5.4A Isat

D3: STPS2L25U: Vf=0.375V, If(av)=2A

I can't see any obvious mistakes I've made, so is anyone able to notice anything that I've missed?  If not, how would I go about debugging a problem like this?

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