data/text/DLP1/899467

LMZM33606: Circuit designed to output 12V is only outputting 1V

Part Number: LMZM33606 Hi All, This is my first step down converter design using SMD parts. So I'm not sure whether its the design or if I am doing something wrong during mounting the SMD parts with a hot air gun. Hope someone can give some advice and point me in the right direction. The circuit is designed to output 12V from a 15V input (well that was the intention anyway). Reading the voltage output on the scope only indicates 1V. I've attached the schematic, board, BOM and also scope readout for reference. Greatly appreciate the help!

Hi Spec, Can you check if FB is shorted to VOUT? The internal voltage reference is 1V and if FB is somehow shorted to VOUT it can force VOUT to be 1V as well. Try probing your board and reading out the voltage measurements to check for any abnormal voltages. Also can you try removing the CFF capacitor and retest board operation? Lastly if you can provide a waveform of VIN, VOUT, and SW that would help troubleshoot the problem. The SW can be accessed on the side of the power module through the SW pins (1-6, 31). Regards, Jimmy

Dear Jimmy, Thanks for the detail pointers. When I set the multimeter to continuity test and probe the FB and VOUT points the sound emitted indicated that those two points are connected. On my actual board as well as my design the FB is connected indirectly to the VOUT through the RFBB, CFF and RFBT and the VOUT plane which the VOUT is connected to. Is this why the FB and VOUT are shorted? When I look back at the reference design in the datasheet it does appear that the FB is connected to VOUT through those resistors and caps. I didn't use a AGND plane in my design so everything is connected to the PGND, perhaps that is why the short between FB and VOUT is happening? FYI, on another prototype the VOUT was fluctuating between 0 V and 2.xx V and did go up to 12V at one point but mostly stayed between 0, 1.xx and 2.xx V. Wonder if this is indicative of the same problem above? Thanks again for your help and looking forward to your feedback. Best Regards, S.

Hi, Jimmy. Here are the waveforms: 1) VOUT: 2) VIN: 3) SW: SW Frequency of peak to peak SMD Solder Paste - just wondering if the solder paste is fully melting?

Hi Spec Pro, Looking the exposed pads of the power module on your PCB, it looks like the solder paste has not completely melted and reflowed properly. A proper reflow of the solder paste should result in the thermal pads being shiny and metallic looking. I would suspect that the device that was on your board didn't have all pins connected and would suggest running the reflow at a hotter temperature to ensure the solder paste is completely dissolved and properly coats the exposed pads. Can you try doing this with a new unit and retest for operation? Regards, Jimmy

Paste is now fully melted and the board is able to convert 15V to 12V. Turns out that heating both sides of the board is necessary to get the paste on the hidden pads to melt as well. Thanks for your help Jimmy!