Part 3: Schematic Capture
Welcome back or if this is your first time here you can check out the rest of the series at Spin It! - Part 1 (Introduction) and Spin It! - Part 2 (Component Selection).
We have our idea in mind, we have our motor to spin and we found the main building blocks for our design….now what? Well now we can start on the more difficult parts, such as schematic capture, layout, and debug/test. You can see below how these fall into a typical PCB design flow. The schematic is the most crucial part of the design. It is, in essence, the blueprint for the circuit.
As with most things, the hardest part is getting started. I recommend starting off by creating all of the schematic symbols you think you might need. You will find taking care of this initially will save time in the end. This includes ICs, headers, connectors, discrete components, etc. For my design I started off by creating a schematic symbol for the DRV8711 (stepper pre-driver IC). I find the most helpful reference for schematic symbol creation is the device pin out figure in the datasheet. Many schematic/footprint symbols have already been created by TI! Check them out here.
*You will see that many motor driver ICs come with a PowerPAD or Thermal Pad. These are utilized to provide a large, low thermal resistance, path for heat from the IC to the PCB. It is a good practice to set this as Pin 0 and connect it to GND unless otherwise specified. You can see this Application Report on PowerPAD for more information.
Now that we have our components, let’s get them connected! I work in blocks when doing schematic design. I will isolate subsections of the system and complete these one at a time, until the design is complete. This helps in more complicated designs as you can reuse these subsections repeatedly.
For example, in this BoosterPack design I identified 5 subsections including:
1) DRV8711 and its external components
2) External power FETs
3) Sense components
4) Power and motor connections
5) LaunchPad connections
Shown below are each of the completed blocks:
1) External components values for the DRV8711 and most ICs can be found in their associated datasheet. For many high-side/low-side N-channel drivers you will see capacitors for the charge pump. The charge pump provides the supply voltage + gate voltage needed to bias the high-side FETs. Many other common components include decoupling capacitors for the internal regulators and pull-up resistors for open-drain outputs. The system will also contain bulk capacitance for the instantaneous current demands of a switching system.
2) The DRV8711 drives 8 N-Channel Power MOSFET’s to create 2 H-Bridges. These in turn will drive the stepper motor. The FETs utilized in this design are dual FETs with two in each package.
3) The DRV8711 maintains current regulation by sensing a voltage across a sense resistor. These low tolerance power resistors allow for precise microstepping.
4) The motor and power connections are made through terminal block headers to allow for quick connection of your motor and power supply.
5) The BoosterPack docks onto the LaunchPad through several headers. These headers supply the control and feedback signals of the MSP430. To ensure compatibility across the various LaunchPads (MSP430, Tiva-C, C2000) I have made BOM population options for the PWM signals.
Finally, when we put these all together, we get a complete hardware system that should, in theory, spin a bipolar stepper motor!
*Another little trick of mine, which you will notice in these images, is that I often use net labels to connect nets together instead of wires. I find this makes the schematic much easier to read and debug.
This may seem like organizational overkill for such a simple design, but you will find, while transitioning to larger designs, these practices will greatly improve your productivity!
Lastly, you can run any schematic/netlist checks your tool might have to search for errors.
Thanks for reading and feel free to leave comments! Next time, we will get into the nitty, gritty aspects of layout design. For more information, you can visit the TI Motor Driver Forums or check out the TI Motor Drive & Control Home Page. If you would like to Build You Own BoosterPack, check out ti.com/byob!
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