[FAQ] Common MCx Schematic and Layout Q/A

In this E2E FAQ we will cover frequently asked questions about MCx devices in schematic and layout stages of design.

1. How should I configure the Thermal Vias on the BLDC motor driver device?

A common equation we use for the thermal resistance of vias is noted here:

where,

Length is thickness of PCB board
Area is (number of vias x (^2−(−)^2)) , where R is radius of vias and t is plating thickness
Thermal conductivity is that of copper: 0.385 W/mm-K

Some conclusions to make from this:

The higher the number of vias, lesser the thermal resistance.
Plan to add as many vias as possible in thermal pad for better heat dissipation. Datasheet layout is only a suggestion – add as many vias as allowed by PCB manufacturer.
It may be better to add more smaller vias than a few larger vias – optimize based on above formula and PCB manufacturer rules

2. How should I configure the GNDs. Do I need a PGND and an AGND?

Use different ground planes (whenever possible) for AGND and PGND to isolate the switching noise from low-voltage signals (referenced to AGND).

Tie PGND and AGND through a 0-ohm resistor; 0-ohm can be replaced with suitable inductor/ferrite bead to isolate high frequency noise while tying the two ground planes for DC signals.

DGND can be tied to AGND plane and GND_BK can be tied to PGND plane.

3. Do I need to connect all three VM and PGND pins to VM and PGND respectively?

Use all three VM and PGND for connecting to respective VM and PGND power Cu planes/pours.

Not using all VM/PGND pins for connections will increase the RDS_ON by approximately 10-15mΩ per pin compared to datasheet values.

4. Should I leave NC pins floating or connect them to GND?

While not required, engineers often connect the NC pins 22-25 to AGND. These pins (circled in yellow below) can be connected to thermal pad and to AGND plane for better heat dissipation: