N00b here, so forgive me if this is a dumb question. I’m using the DC-DC converter VXO7803-500, which comes in a 3-SIP package. There were no compatible symbols/footprints in the standard library, but SnapEDA had one, which I downloaded. I’ve had a really hard time getting the ERC happy with it and the PWR_FLAG, and I’m not sure that the way I’ve solved it is right.
This is the recommended schematic for my application:
[will put image in comment because I’m a new user and it won’t let me attach two images as a new user]
The complication is that this converter can be connected so that its output is either +3.3V or -3.3V, and the way you accomplish negative voltage is to swap the VOUT pin and the GND pin and then reverse the polarity of C2. In other words, Pin 3 is either +VOUT or GND_IN_OUT and Pin 2 is either GND_IN_OUT or -VOUT.
I managed to get the ERC errors to go away by editing the symbol and changing Pin 1 to “Power input” and both of Pins 2 and 3 to “Power output” (even though I’m using Pin 3 as the power output and pin 2 as the common pin, which is neither input nor output) and then putting the PWR_FLAG on Pin 1. I tried making Pin 2 “bidirectional,” but that just created different ERC errors.
Did I solve this correctly? Is there a better way?
As an aside, now I have the ERC warning “Symbol ‘VXO7803-500’ has been modified in library ‘VXO7803-500’.” I figure I can probably ignore this, but should I have edited this differently? (I imported the library into my project specific libraries, not into the global libraries.)
I don’t care about power_flag as far as I know what I want to connect.
Signals entering a power input pin must come from a power output pin of another device. When this is not the case, for example the power comes from a connector or through an inductor, the power_flag is used to tell the input power pin that the signal is a power one and keep tje ERC happy. The power_flag has no electrical effect on the circuit.
Usually I set the GND pins as input power pins. In your case I would ignore the error or I would make 2 symbols, one for “positive” and one for “negative” outputs.
I don’t use ERC so can’t be sure of what I write but I think it is wrong idea to make GND pin being “Power output”. Technically nothing prevents you from using 2 such converters at one PCB and in such case their GNDs will be connected together (I assume both being positive supply). If two “Power output” pins were connected together than ERC probably will not like it.
The same problem will be if you use one as positive and second as negative.
And what if other power supply ICs will also have their GND flagged as “Power output”.
Other subject.
I think that searching for a symbol and then wage war with it is generally wrong way. Learn how to make your own libraries and then in your library you will just copy another power supply IC symbol to get this one. When I planned to use KiCad first what I have done was to make my own libraries and I use only them. Making symbol or footprint is much simpler task than designing PCB.
I can’t tell more about how to make your own libraries as I was doing it with KiCad V4 and it was completely different than now. I decided which libraries I want to have (in some of them I put dummy element (symbol/footprint) only to have there something until I put there first real element when O removed that dummy. Since than I don’t make new libraries. I only “Save as” one symbol as another.
I went to Preferences → Manage Symbol Libraries, then to the Project Specific Libraries tab, then clicked on Add Existing Library to Table (folder icon), then selected the .kicad_sym file that I downloaded from SnapEDA. That’s then what I edited.
In retrospect, you’re right. Still a n00b here. Didn’t know anything about creating my own symbols when I started this journey. But, yeah, really, there should be two symbols for this DC-DC converter: One in the +V mode and one in the -V mode. I could use a custom symbol for this and then still use the correct footprint that I got from SnapEDA.
The warning occurred because you have incorrectly imported then modified the SnapEDA symbol.
The correct method is to:
Create a Personal Library
Import the symbol into that Personal Library.
Open the symbol in the Personal Library
Modify, save, then use that Symbol on the Schematic.
This FAQ describes how to make your Personal Libraries.
After you have made a library:
Download your required symbol
Open your Symbol Editor.
Highlight your new personal library.
Click File > Import Symbol
Navigate to your Downloads and select the symbol and Save.
Close that screen.
Open the Symbol Editor, then your Personal Library, then your Imported Symbol and finally modify that symbol, save and then use that symbol in your Schematic.
You will no longer have warnings regarding that symbol
