Tool/software: WEBENCH® Design Tools
So I designed a 5V supply using LMR33630ADDR, switching frequency is 400kHz. We expect the input voltage would be as high as 36V, since that is what the chip is rated. In testing, the chip would fail at 36V input. In 24V it works totally fine.
The chip will fail in a conductive way, sending 36V down the line and killing every downstream devices. The failure is instantaneous, at the moment any load is applied on the chip (we use a small 5V, 170mA fan as load to begin with). Here is the schematic of the power supply part:
The design mostly follows the reference design in the datasheet, except:
Four small capacitors are merged into a large one, to save space. What is the benefit of using 4 small capacitor in parallel instead of one large one anyway?
Inductor is much larger than the reference, but that's what I got from the formula in datasheet : L = [(Vin-Vout)/(fsw*K*Ioutmax)]*Vout/Vin, where Vout is 5V, Vin is 36V, fsw is 400k, Ioutmax is 3A and K is 0.2. This would yield an L of 18uH(17.9398u), using 15uH due to space limitation, yeilding K between 0.2 and 0.3. It seems both WEBENCH and some form suggestion here never suggested any inductor larger than 10uH, I woulder why.
PCB layout also follows the datasheet suggestion. This is a reflowed PCB so there shouldn't be a problem on ground pad contact.
Any suggestions would be greatly appreciated.