hi I’m having a bit of an issue with my PCB board, I have a 3.3V power plane but, I also need 5.0 volts for 1 component.
I’m using 2 voltage regulators an LM1117-3.3v and LM1117-5.0V. I attempted to connect the input of each voltage regulator to the power supply and then use a common ground, with just a trace supplying the component that needs 5 volts, and the 3.3v output supplying the power plane.
Unfortunately my 3.3v power plane is now 5volts, can anyone show me the proper way to use 2 voltage regulators on a circuit?
mcnscott:
hi I’m having a bit of an issue with my PCB board, I have a 3.3V power plane but, I also need 5.0 volts for 1 component.
I’m using 2 voltage regulators an LM1117-3.3v and LM1117-5.0V. I attempted to connect the input of each voltage regulator to the power supply and then use a common ground, with just a trace supplying the component that needs 5 volts, and the 3.3v output supplying the power plane.
Unfortunately my 3.3v power plane is now 5volts, can anyone show me the proper way to use 2 voltage regulators on a circuit?
Thank you
Scott
Without your schematic, it sounds as if you didn’t put any capacitors in the circuit.
If your using LDO’s it is pretty easy. Each has a VIN (voltage in)/GND(common)/Vout(voltage out).
You could have each getting their supply from your power supply, or have the 3.3 getting it’s power from the 5 volt reg.
There are specifics about power draw. If you do not draw enough, some regulators can’t work.
There are many things that could be wrong with your circuit. It is better for you to post a diagram of what you did, and let us correct it. That way you will never forget the mistake you made. I find, I never forget previous mistakes.
You are doing it correctly. I would look for a short between your 5V regulator output and the power plane. If you find none, try removing your single 5V component and see if the problem still exists.
thanks for the replies! I’ve included a snippet of my schematic and my pcb design, I think my problem may be with c2 and c5, they are going straight from power to cap to ground, is that wrong? Looking at my design I probably should move my caps, they don’t seem to conform to my schematic.
The power supply is 9v 800mA, and is coming through the diode and then the capacitors then splits at the 2 regulators. Again I think my capacitor placement may be the problem
mcnscott:
thanks for the replies! I’ve included a snippet of my schematic and my pcb design, I think my problem may be with c2 and c5, they are going straight from power to cap to ground, is that wrong? Looking at my design I probably should move my caps, they don’t seem to conform to my schematic.
The power supply is 9v 800mA, and is coming through the diode and then the capacitors then splits at the 2 regulators. Again I think my capacitor placement may be the problem
Thanks for the assistance
Scott
On the LM1117 the tab is V-out. Is your tab connected to the plane? If so, there is your problem.
Your schematic looks correct, and so does the board. It is hard to tell about the tabs. Some manufacturers connect the tab different. (Like Micrel)
Where does the via just above C5 go? I do not see a connection for it. May just be elusive.
Thank you for looking at the board, I wonder if the short is the C6 capacitor because I had to really squeeze that capacitor on the trace. The via above C5 goes to ground.
Should I be using the tab as my voltage out? I was just using pin 2. I have the tabs isolated on their own pads with no connections to anything.
mcnscott:
Thank you for looking at the board, I wonder if the short is the C6 capacitor because I had to really squeeze that capacitor on the trace. The via above C5 goes to ground.
Should I be using the tab as my voltage out? I was just using pin 2. I have the tabs isolated on their own pads with no connections to anything.
Thank you
Scott
It is a good idea to use the tab, but not required.
I notice you have the 5 volt regulator (GND) going to a header, is that header pin also common to ground?
Not sure where the problem is, but your circuit does look correct. I would take a meter and check for shorts on the 5 volt part of the circuit. Also check the header pin to insure it is actually connected to the common ground.
The only other possible source would be somewhere else in the 5 volt circuit, but I suspect you have that part disconnected.
Thank you for looking at the board, I wonder if the short is the C6 capacitor because I had to really squeeze that capacitor on the trace. The via above C5 goes to ground.
Should I be using the tab as my voltage out? I was just using pin 2. I have the tabs isolated on their own pads with no connections to anything.
I just thought about another thing that could be happening, even if you have no shorts. Are you by chance tying the output from your 5V part to an input of a 3.3V part? If you are, and are relying on the clamping diodes inside your 3.3V input, that could be your problem. When your 3.3V side uses little current, it is possible to raise the 3.3V power rail up to near 5V through the clamping diodes. In essence, you would have your 5V supply feeding into your 3.3V supply through a single diode.
Out of curiosity, why is your copper pour so tight to your traces? Are you milling this board? I would personally set a larger offset, especially if you don’t have a soldermask.
