Single-chip driver
The first option is to use a monolithic driver IC to drive the motor. An integrated IC consists of a single silicon chip in a package; this chip integrates logic, support and protection circuitry, as well as power devices such as MOSFETs that drive current through the motor.
Because the MOSFETs and control circuitry in monolithic solutions are fabricated on the same chip, these solutions offer the advantage of accurate current measurement. Monolithic ICs also provide robust protection features such as overcurrent protection (OCP) and overtemperature protection (OTP) because the circuitry can be placed near the MOSFETs on the silicon chip.
Integrated drivers are limited to rated voltages and currents compatible with IC processes, meaning the highest available rated voltage is between 80 V and 100 V. Furthermore, these drivers can drive up to approximately 15 A.
Single-chip drivers are almost exclusively used in high-capacity applications such as printers, where the power supply voltage is typically below 35 V and the motor current is below 5 A.
An example of an integrated driver is the MPQ6541, a 3-channel power stage device specifically designed for automotive applications. It is rated for supply voltages up to 45 V and continuous load current of 8 A, or peak current of 15 A per channel. This motor driver integrates six MOSFETs, each with an RDS(ON) of 15 mΩ. It is packaged in a TQFN-26, 6 mm x 5 mm flip-chip package.
Gate driver
The second option is to use discrete power MOSFETs (or in some cases, other power devices) to drive current through the motor, and the MOSFETs are controlled by a gate driver IC, a pre-driver, or multiple gate drivers.
For applications requiring high voltages exceeding 100 V or very high currents, there is no monolithic solution. In these cases, a gate driver and discrete MOSFETs are required.
Because multiple devices are required in this case (sometimes up to three gate drivers and six power MOSFETs), the solution size (i.e. the PCB area occupied by the motor driver) is much larger than the size required for a single driver.
An example of a highly integrated gate driver is the MPQ6533, a 3-channel gate driver IC with integrated features such as slew rate control and internal diagnostics. This device is packaged in a 5 mm x 5 mm QFN-32 package.
Cost considerations
Analog and mixed-signal IC processes are far more complex than dedicated discrete MOSFET processes. Because manufacturing low RDS(ON) MOSFETs in IC processes requires a large area of silicon, devices with the same RDS(ON) and voltage in MOSFET processes typically cost more than those manufactured using dedicated discrete MOSFET processes.
For motor drivers with lower current and/or lower voltage, the cost of fabricating MOSFETs in an IC process is minimal. Since control and protection functions occupy a large portion of the chip, the added area for MOSFETs does not increase cost as much as using external MOSFETs.
However, for high-current applications, the cost of MOSFETs in the IC process begins to dominate the device cost. Although monolithic motor drivers are available that can support 15A of motor current, they are typically more expensive than implementations using gate drivers and discrete MOSFETs.
In some cases, the small size of a single component is so highly valued that it justifies a more expensive solution. For example, some systems require integrating a driver inside a motor, but the available space is limited. In these situations, solutions using gate drivers and MOSFETs may simply not be suitable for the confined space.
To get a rough idea of the relative cost of a monolithic solution versus a gate driver solution, we can compare the cost of a monolithic IC plus a gate driver IC with three dual MOSFETs and three current-sensing resistors. Other supporting components (such as bypass capacitors) are similarly priced between the two solutions. Note that these costs are based on small catalog prices; actual mass production prices are typically much lower.
Solution size
Monolithic drivers are almost always smaller than equivalent solutions using gate drivers and discrete MOSFETs.
For example, we can compare the PCB area occupied by the MPQ6541 and the MPQ6533, as well as the additional power MOSFETs. The two parts differ significantly in size; the MPQ6541 occupies 130 mm², while the MPQ6533 occupies 520 mm², four times the size. Note that the gate driver solution shown here uses dual MOSFETs in a small package; in other cases, the MOSFETs can be larger, further increasing the PCB area of the solution.
Heat dissipation precautions
To effectively dissipate the heat generated in power MOSFETs, the PCB is typically used as a heatsink. Larger packages generally have better thermal conductivity on the PCB, meaning that larger solutions are better from a heat dissipation perspective. This is advantageous for solutions using gate drivers, as power MOSFETs are typically large. Low R<sub>DS(ON)</sub> power MOSFETs are readily available, so in some cases—especially applications requiring operation in harsh environments—thermal factors may preclude the use of monolithic drivers.
Monolithic drivers use smaller packages. To compensate for the higher thermal resistance in these packages, the R<sub>DS(ON)</sub> for a given current must be lower than that of a similar solution using discrete MOSFETs.
Consider the MPQ6541 monolithic driver and its small size. With proper PCB design, this section can drive significant currents. The temperature of the MPQ6541 on a 5 cm x 5 cm, 2-layer PCB is shown, while supplying 6 A current to a three-phase brushless motor. The measured case temperature is 38°C higher than ambient temperature. A 4-layer PCB with internal planes would further reduce temperature rise.
Careful consideration and weighing
Choosing between a monolithic motor driver and a gate driver that uses an external MOSFET solution to drive the motor is complex. Trade-offs between cost, solution size, and thermal characteristics must be considered.
For very small motors, a monolithic driver is the optimal solution. Similarly, for very high-power motors, a solution using gate drivers and discrete MOSFETs should be used. However, there is significant overlap between these two solutions, so designers should consider the specifications of their application when making a choice.
