EAGLE, Complex Device, And Vias - Stepper Driver Discussion
idbruce
Posts: 6,197
Over the years, I have worked on designing various stepper drivers, but never followed through on any of the designs. With new goals and direction, I want to start designing another stepper driver, but with a different chip. I want to base this driver around the TI DRV8825, which will be a serious challenge, because I have never used surface mount parts in any of my previous designs.
My design goals are as follows, and of course this is a wish list (depending on IO requirements:
1. Take full advantage of the DRV8825's amperage, operating voltage, and various settings
2. To enable a wide range of voltage, I will be using a LM5010A switching regulator on board
3. Dedicated Propeller chip on board
4. Provisions for a couple of pushbuttons
5. Provisions for a couple of LEDs
6. Provisions for a rotary or linear encoder
7. Reduce input voltage down to 3.3V for powering the Propeller
8. Provisions for inter-Propeller communication
9. Provisions for piezo siren
10. Provisions for micro-SD
11. Provisions for a couple of various sensors
12. Provision for a small amount of prototyping
Anyhow... I am currently designing a complex device in EAGLE for the DRV8825, and I now have all the pads set, including the "Solder Mask Defined Pad" for the "PowerPad Thermally Enhanced Package". Before going any further with the design of this device, I have a question. In the datasheet of the DRV8825, the layout of the "Solder Mask Defined Pad" shows 21 vias to help get rid of the heat which is generated from the driver. My question is, if I put these vias in the design of the device, will these vias transfer to the bottom of a PCB when added to a PCB design, or should I add these vias later after the board has been generated?
My design goals are as follows, and of course this is a wish list (depending on IO requirements:
1. Take full advantage of the DRV8825's amperage, operating voltage, and various settings
2. To enable a wide range of voltage, I will be using a LM5010A switching regulator on board
3. Dedicated Propeller chip on board
4. Provisions for a couple of pushbuttons
5. Provisions for a couple of LEDs
6. Provisions for a rotary or linear encoder
7. Reduce input voltage down to 3.3V for powering the Propeller
8. Provisions for inter-Propeller communication
9. Provisions for piezo siren
10. Provisions for micro-SD
11. Provisions for a couple of various sensors
12. Provision for a small amount of prototyping
Anyhow... I am currently designing a complex device in EAGLE for the DRV8825, and I now have all the pads set, including the "Solder Mask Defined Pad" for the "PowerPad Thermally Enhanced Package". Before going any further with the design of this device, I have a question. In the datasheet of the DRV8825, the layout of the "Solder Mask Defined Pad" shows 21 vias to help get rid of the heat which is generated from the driver. My question is, if I put these vias in the design of the device, will these vias transfer to the bottom of a PCB when added to a PCB design, or should I add these vias later after the board has been generated?
Comments
Both methods are commonly used. ( Any thru hole will transfer to the bottom, and also inner layers if you have them)
Thermal-Vias-in-part has the benefit they cannot be deleted by any over-zealous DRC, nor moved during routing.
It does mean you cannot 'sneak space' by remove or nudge of a via or two, and you need to decide now, how many to use.
It may also mean you need to connect those 'via-pins' in the Sch side, to make DRC happy.
Some PCB packages have hidden pins for this purpose.
Thanks for the answer.
My current intention is to layout it out exactly as described and shown in the datasheet, with all 21 holes, so hopefully I can obtain the max current per phase. However I realize there can be a benefit by not striving for max current, such as more maneuvering room for traces. Perhaps I should make two packages, one with the thermal pad designed in and one with the thermal pad designed out, to add later.
I do appreciate your input. Thank you.
You could also consider 4 layer PCB ( even 2oz) ?
If the PCB is not large, the cost adder these days for 4 layer is more tolerable, and you get twice the copper-density of 2L designs, plus more routing channels.
I am using the free EAGLE version. If I am not mistaken, I believe it is limited to two layers of copper. I never really thought too much about multi-layer boards. I don't know if my wee brain could handle that and surface mount
However the datasheet did mention thermal disipation for multi-layer boards.
KiCad looks very interesting. What is the learning curve on the software? Easy or hard?
All CAD tools have their quirks....
If you know Eagle already, I'd suggest checking the Eagle->KiCad import of some boards you have done already, plus a dummy board with that new footprint and some tracks etc.
If that looks to work ok, you can use Kicad as the final stages tool - it has a very nice Shove Router, copper Pour is good, and it can do 4 layers easily if you choose that.
That flow keeps SCH and Footprint Library work in Eagle, and less to learn in kiCad.
Hello Bruce, nice to see you coming back, sad to hear the reason that hold you away.
We created a stepper driver board, currently we have revision A boards. The Trinamic drivers have some special features like: direct access to current registers, so you can create any output currents, stall detection to show overload and automatic current adjustment to lower temperature of the motor. This board has 4 channels, everyone has limit switches and encoder support. The board can be driven > 24 V and creates 5V and 3.3 internally. A RS485 interface is provided with galvanic insolation. The board fits to the quick-start format, different units can be stacked. No siren ;-(
I am basically starting this board for projects that require a one motor solution, such as a label dispenser, so I will be aiming for the smallest board possible, with the previously mentioned wishlist in mind. Most likely, many of the supporting requirements will not be used, but I just want them, in case I need them for any particular project.