Motor Drive Design: The Difference Between Integrated Drivers and Gate Drivers

When designing a motor control circuit, it is critical to determine how to provide the high currents required to drive the motor. Designers must choose whether to use a monolithic integrated circuit (IC) with internal power devices or to use a gate driver IC and discrete external power MOSFETs.

This article discusses the pros and cons of each approach and provides guidance on when to choose between the two solutions.

Monolithic Drivers

The first option is to use a monolithic driver IC to drive the motor. An integrated IC consists of a silicon die in a package; the chip integrates logic, support, and protection circuits, as well as power devices such as MOSFETs that drive current through the motor.

Because the MOSFETs in monolithic solutions are fabricated on the same die as the control circuitry, these solutions offer the advantage of accurate current measurement. Monolithic ICs also offer robust protection features such as overcurrent protection (OCP) and overtemperature protection (OTP) because the circuitry can be placed close to the MOSFETs on the silicon die.

Integrated drivers are limited to voltage and current ratings that are compatible with the IC process, which means that the highest voltage ratings available are between 80 V and 100 V. Furthermore, these drivers can drive up to about 15 A.

Monolithic drivers are used almost exclusively in high-volume applications such as printers, where supply voltages are typically less than 35 V and motor currents are less than 5 A.

An example of an integrated driver is the MPQ6541, an automotive-specific 3-channel power stage. It is rated for supply voltages up to 45 V and continuous load currents of 8 A, or peak currents of 15 A per channel. This motor driver integrates six MOSFETs, each with an R DS(ON) of 15 mΩ. It is packaged in a TQFN-26, 6 mm x 5 mm flip-chip package.

Gate Drivers

The second option uses 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, pre-driver, or multiple gate drivers.

For applications that require high voltages in excess of 100 V or very high currents, no monolithic solution exists. In these cases, a gate driver is required along with discrete MOSFETs.

Because multiple devices are required in this case (sometimes as many as three gate drivers and six power MOSFETs), the solution size (i.e., the PCB area occupied by the motor driver) is much larger than that required for a monolithic 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. The device is available in a 5 mm x 5 mm QFN-32 package.

Cost Considerations

Analog and mixed-signal IC processes are much more complex than dedicated discrete MOSFET processes. Because a large area of ​​silicon is required to fabricate a low R DS(ON) MOSFET in an IC process, the cost of a device with the same R DS(ON) and voltage in a MOSFET process is generally higher than the cost of fabricating a similar device in a dedicated discrete MOSFET process.

For lower current and/or lower voltage motor drives, there is little penalty for fabricating the MOSFET in an IC process.

Because the control and protection functions occupy a large portion of the chip, the added area for the MOSFET does not increase the cost as much as using external MOSFETs.

However, for high-current applications, the cost of the MOSFETs in the IC process begins to dominate the cost of the device. Although there are monolithic motor drivers that can support 15A of motor current, they are generally more expensive than implementations using gate drivers and discrete MOSFETs.

In some cases, the small size of a monolithic part is so highly valued that it justifies a more expensive solution. For example, some systems require the integration of a driver inside the motor, but the available space is small. In these cases, a solution using a gate driver and MOSFETs may simply not fit into the constrained 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 sense resistors. Other supporting components, such as bypass capacitors, have similar prices between the two solutions. Note that these costs are based on low-volume catalog prices; actual volume production prices are typically much lower.

Solution Size

A monolithic driver is almost always smaller than an equivalent solution using a gate driver and discrete MOSFETs.

For example, we can compare the PCB area occupied by the MPQ6541 versus the MPQ6533, with the additional power MOSFETs. The size difference between the two parts is significant, with the MPQ6541 occupying 130 mm2 and the MPQ6533 occupying four times the area at 520 mm2. Note that the gate driver solution shown here uses dual MOSFETs in a small package; in other cases, the MOSFETs can be larger, which further increases the solution’s PCB area.

Thermal Considerations

To effectively dissipate the heat generated in the power MOSFETs, the PCB is often used as a heat sink. Larger packages generally have better thermal conductivity to the PCB, which means that larger solutions are better from a thermal perspective. This favors solutions using gate drivers because power MOSFETs are typically large. Low R DS(ON) power MOSFETs are readily available, so in some cases—particularly applications that need to operate in harsh environments—thermal considerations may preclude the use of monolithic drivers.

Monolithic drivers come in smaller packages. To compensate for the higher thermal resistance in these packages, the R DS(ON) for a given current must be lower than the R DS(ON) of a similar solution using discrete MOSFETs.

Consider the MPQ6541 monolithic driver and its smaller size. If the PCB is designed correctly, the part 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 to a 3-phase brushless motor. The measured case temperature was 38°C above ambient. A 4-layer PCB with internal planes will further reduce the temperature rise.

Consider the Tradeoffs Carefully

Choosing between a monolithic motor driver and a gate driver that drives a motor with an external MOSFET solution is complex. The tradeoffs between cost, solution size, and thermal characteristics must be considered.

For very small motors, a monolithic driver is the best solution. Likewise, for very high power motors, a solution using a gate driver and discrete MOSFETs should be used. However, there is a large overlap between the two solutions, so designers should consider the specifications of their application when making a choice.