Mini usb 5v to 3.3v PCB

[justind000]

I've got a project I'm working on that requires using a mini usb connection to supply various components with either 5v or 3.3v. This board is my first real PCB. After making it myself, everything seems to look ok. Checking the output and ground pins shows the expected voltages, but as soon as it's connected to the usb, it blows the transistor.



From left to right, the components are a mini-usb connector, a 1uf cap, a XC6206P332MR (662K) 3.3V/0.5A SOT-23 SMD triode transistor, and another 1uf cap. The pins on the top are 2 5v, and 2 3.3v and the pins on the bottom are ground.

I'm sure it's a fairly obvious fix. Thanks.

 

[kam1787]

a schematic would make it easier ................

 

[justind000]

a schematic would make it easier ................

Here it is. The trans is actually a 662k, not whatever the eagle part was.

 

[kam1787]

I am a little confused, because you talk about a transistor but the part numbers are for an IC. How much current is the load pulling? Otherwise I don not see a problem.

 

[justind000]

I am a little confused, because you talk about a transistor but the part numbers are for an IC. How much current is the load pulling? Otherwise I don not see a problem.

I couldn't find an eagle part that was the right footprint until I stumbled onto that particular part. It's actually this
XC6206P332MR (662K) 3.3V/0.5A SOT-23 SMD triode transistor

It's powering an arduino pro mini, an NRF24 radio, and a sensor or two. I don't have a MM that measures current, but from reading, it should be somewhere around 150 mA max.

I've been learning things as I go. If the circuit looks correct, I can only assume that I'm not assembling it correctly, but the last board I made, I checked continuity after placing each part and everything seemed ok. The two 5v pins and two 3.3v pins read as expected, but when it's connected to a load, it blows the transistor. I have other applications that use the same transistor in different circuits that have been working just fine for months now (with the arduino, radio, and sensors).

Thanks for your time.

 

[kam1787]

It is not a transistor. It is an IC

Have you tried a dummy load - say a 330 ohm resistor to start?

 

[betwixt]

It isn't a "triode transistor" (whaever that is) its a voltage regulator. Assuming the PCB has all components top-side it looks OK. Maybe the current you are drawing is overheating the regulator - check it's dissipation, you might have to mount it on a larger copper area to spread the heat.

Incidentally, the maximum output current for that device is 200mA. The figure you quote is it's absolute maximum rating for the 5V version.

Brian.