Motor-driver IC PCB layout guidelines, Part 2

August 10, 2017 // By Pete Millett, Monolithic Power Systems (MPS)
In Part 2 of this article, we explore best PCB-layout practices for a variety of motor-driver package types.

Part 1 of this article (read it here) provided some general recommendations for designing printed-circuit boards (PCBs) using motor-driver ICs, which require careful PCB layout for proper performance. Part 2 discusses some specific PCB layout recommendations for using typical motor-driver IC packages.


Layout for leaded packages



Figure 6. SOT-23 and SOIC packages are typically used in low-power motor drivers.


Standard leaded packages, such as SOIC and SOT-23 packages, are often used for low-power motor drivers (Figure 6).


To maximize the power-dissipation capability of leaded packages, Monolithic Power Systems (MPS) uses a “flip-chip on leadframe” construction (Figure 7). The die is bonded to the metal leads using copper bumps and solder without the use of bond wires. This allows heat to be conducted from the die through the leads to the PCB.



Figure 7. A flip-chip on leadframe structure helps maximize power dissipation in leaded packages.


Thermal performance can be optimized by attaching large copper areas to the leads that carry high current. On a motor-driver IC, typically the power, ground, and output pins are attached to the copper areas.


Figure 8 shows a typical PCB layout for a “flip-chip on leadframe” SOIC package. Pin 2 is the device power pin. Notice that a copper area is placed near the device on the top layer, and several thermal vias connect this area to copper on the backside of the PCB. Pin 4 is the ground and is connected to the copper ground pour on the top layer. Pin 3, the device output, is also routed to a large copper area.



Figure 8. Shown is a flip-chip SOIC PCB layout.


Note that there are no thermal reliefs on the SMT pads; they’re solidly connected to the copper areas. This is critical for good thermal performance.


QFN and TSSOP packages


TSSOP packages are rectangular in shape and use