Proper Power and Ground Design

[bryan]

Hello All:

I have a circuit consisting of a PIC that provides a clock and other control TTL level voltages for a L297 stepper controller which in turn drives a L298 stepper driver. The power for the whole circuit is provided by a 12v switching power supply. The supply directly feeds the L298 for the stepper motor. The L297/L298 and PICc are powered by a 5 volt regulator off the 12v power supply.

Seems to work well but I want to redo the design a bit, I do have some concerns about the amount of noise on the power lines from the Stepper motor. I have the appropriate bypass caps everywhere, but wondering if there is some tips to limit the power supply noise. Should I perhaps use 2-5volt regulators off the 12v supply. One for the L297/298 and another one for the PIC.

I have heard topics such as star grounds.??

Any suggestions or tips appreciated

Cheers

 

[ronv]

Kinda like this: Make the ground pin on the regulator the ground point for all the logic (pic & 297). Make the ground where it enters the board the ground point for the Power grounds (298 sense resistors).
Decouple the input with an electrolytic and ceramic cap where it enters the board and decouple each IC with a ceramic Add a large low ESR cap to the 12 volts in the LM298.
So the star is the regulator ground and output for the logic and the input to the board for the 12 volt power and ground.

 

[crutschow]

Make sure none of the stepper motor ground or driver current runs through any part of the driver input or PIC circuit grounds. Any of this return current through the electronic circuit ground will generate ground-bounce noise that will be difficult to filter.

 

[bryan]

Kinda like this: Make the ground pin on the regulator the ground point for all the logic (pic & 297). Make the ground where it enters the board the ground point for the Power grounds (298 sense resistors).
Decouple the input with an electrolytic and ceramic cap where it enters the board and decouple each IC with a ceramic Add a large low ESR cap to the 12 volts in the LM298.
So the star is the regulator ground and output for the logic and the input to the board for the 12 volt power and ground.

Thanks for the advice, so just to be sure I understand. All the grounds for the L297 and PIC should terminate at the voltage regulator ground which in turn would also connect to the 12v input ground. The current sense resister grounds should terminate at the 12v input ground. Should the ground for the L298 also terminate at the 12v input ground or to the regulator ground?

Should all the grounds for the logic ie L297 and PIC terminate in a star at the regulator ground and not connect to each other prior to the regulator ground termination.?

 

[bryan]

Make sure none of the stepper motor ground or driver current runs through any part of the driver input or PIC circuit grounds. Any of this return current through the electronic circuit ground will generate ground-bounce noise that will be difficult to filter.

Thanks, I believe that is what ronV is suggesting as well.

 

[ronv]

Yes to both questions. Think of it as a power ground and a logic ground tied together only at the source.

 

[bryan]

Thanks ronv, not sure about the L298, should it terminate at the input ground or the regulator ground?. What about the +5v for the logic should they terminate to the regulator in a star or can they daisy chain off each other?

 

[ronv]

The 298 should terminate at the input ground (power ground) The logic should terminate at the regulator ground, but they are not quite as important because the current is low. What you don't want to do is put the 298 ground at the end of the logic ground in a daisy chain.
Ahh you ask about the +5 not +5 ground. If your a perfectionest the regulator output is best, but since you used a star ground you now have a good ground for the decoupling capacitors to work against. Again usually the currents are small in the logic so daisy chains are often used to simplify the layout.

 

[bryan]

Thanks for all the help

 

[bryan]

Just occurred to me I usually use a ground plane for my PCB's, but from what I have learned from the comments, I should not do so with this project and design the traces on the PCB as discussed.

 

[ronv]

The ground plane is a good idea because the resistance and inductance is lower than just a trace. You might consider a split ground plane: 1 for the logic and 1 for the 298 with the 2 comming together at the input.

 

[Mikebits]

As already mentioned. Two ground planes, one for digits, and other for analog, only tied at source. One thing not mentioned, ensure there is separation between planes to avoid coupling of the planes.