Power Supply Troubles

[wackazong]

Hey,

I ran into some serious issues with my board design today, maybe somebody here can help me out.

I have various modules in my MIDIBox-based controller, which are supplied by 24V from a AC/DC converter. On each PCB, a step down-converter (Texas Instruments TPS5420) converts the 24V DC to 5V DC. This works beautifully. I designed the PCB exactly like it is proposed in the datasheet, and followed the parts advisor in the Texas Instruments SWIFT tool. The 5V come out ok.

To supply my MIOS32 core, I then convert the 5V down to 3.3V using a linear converter (ST Microelectronics LK112SM33TR). All wired like proposed in the datasheet (this one is easy).

Now, if I supply my board directly with 5V (for testing purposes I just get 5V directly from the USB port), everything works, and the core gets 3.3V. This has been working for a couple of weeks now.

Then I plugged in the 24V power supply. Things worked for a couple of hours. Now suddenly I have only about 0.2V coming out of the linear regulator, instead of 3.3V, and it gets extremely hot as soon as I plug in the 24V.

Does anybody have an idea why 5V coming from the step down converter is different to 5V coming from a USB port? I changed to linear regulator and put in a new one, but that did not change anything. Should I maybe take another linear regulator model?

Thanks for any pointers,

ALEXander.

[lylehaze]

I'll take a few guesses..

You did not say whether the USB power worked AFTER the regulator seemed to die.

If the 5 to 3.3 regulator still works on USB power but fails when powered from the 24 to 5 step-down, then you might need to look at the 24 to 5 step-down circuit. First thing to check might be the output filter capacitor. I see they specified tantalum, those can get tricky if damaged..

If the 3.3 regulator does NOT work on USB power after the failure,  then it has been damaged.

The most likely cause would be the output filter capacitor of the 24 to 5 volt step-down circuit.

Either way, I know where I'd be looking first. :-)

If you really want to know what's going on, a scope would tell everything rather quickly.

Unfortunately, not everyone has one.

Good Luck,

LyleHaze

[madox]

...I know where I'd be looking first...

Agreed. Sounds like you have a short. This can happen really easily, and a blown cap or transistor/reg that has opened up can cause it. It could be a barely perceptable wire strand or solder tag, also. I would be testing impedances first, before plugging it back into your USB port. Impednace tests can be misleading, but are not bad as a quick check.

If you have already run the board from USB after the supply failure, then do post back with results.