Determining supply voltage, output current, and motor power in a system is one of the first steps in selecting what type of motor driver architecture is needed for an application.

Supply voltages come from two categories: battery powered and line powered. In both battery and line powered systems, the supply can vary in voltage, so a motor driver should support at least the maximum voltage of the battery with extra headroom in the case of voltage feedback or transients in the system. TI recommends using a motor driver rated up to 1.2 × the maximum voltage for well-regulated supplies and low-power motors, and 1.5 to 2 times for high-power motors and battery systems. Texas Instruments has a wide-ranging portfolio of motor drivers that support up to 56-V battery systems.

In general, integrated versus external FET architectures have different power requirements. High power (> 70W) systems use gate drivers and low-to-mid-power systems (< 70W) use integrated FET drivers. External FETs are able to drive higher power than integrated FETs because they are not constrained by the size of the single-chip integrated FET driver device. For integrated FET solutions, peak current, RMS current, and R_DS(on) of the internal FETs are important considerations that directly relate to the motor power. For external FET solutions, the R_DS(on) and current ratings of the external MOSFETs relate to the power the motor can drive.

• Integrated FET
  • Motor power for integrated FET architectures can be calculated by Equation 1, where VM is the motor voltage and I_RMS is the nominal current of the motor.
    Equation 1. $\mathrm{P} = \mathrm{VM} \times {\mathrm{I}}_{\mathrm{RMS}}$
  • Peak current is the maximum short duration current in a motor that can be caused by switching, inrush, or parasitic effects. Many motor drivers today have built in protection such as overcurrent protection. The peak current is the maximum current that can be driven before overcurrent protection kicks in. TI’s Integrated FET drivers can drive up to tens of amps in peak current.
  • RMS current (or continuous current) is the nominal current of the motor and directly relates to the power dissipation of the motor.
  • For high-power systems, it may be difficult to find an integrated FET driver to meet peak and RMS current specifications, which means that the system needs to use a gate driver instead of an integrated FET driver.

• Gate Driver + External FET:
  • External FET architectures can drive much more power than internal FET architectures because of the lower R_DS(on) of external FETs. The larger size of external FETs allows their R_DS(on) to be much lower without affecting motor driver die size. For example, an internal device may have an R_DS(on) of hundreds of milliohms while an external FET may have less than 10 mΩ.
  • Gate driver current is the current supplied to the gates of the external MOSFETS, which controls the rate of ON/OFF switching. Although not directly related to motor power, it is an important consideration as it relates to the slew rate, EMI performance, and thermal performance of the MOSFETs. TI gate driver architectures can source up 3.5-A of current and sink up to 4.5-A of gate driver current.
  • The relationship between gate drive current and rise time to switch the FET on is calculated in Equation 2, where Q_GD is the gate-to-drain capacitance of the FET (which is the major contributor of the VDS slew rate of the FET) and IDRIVE is the gate drive current.
    Equation 2. ${\mathrm{Q}}_{\mathrm{GD}} = \mathrm{IDRIVE} \times {\mathrm{t}}_{\mathrm{rise}}$
  • If IDRIVE gate current is too high, it can cause overshoot, undershoot, or switch-node ringing that negatively affects EMI performance. Conversely, if IDRIVE gate current is too low, thermal losses can increase in the MOSFETs due to power dissipation from switching losses, where the motor current is continuing to flow during the MOSFET saturation region.
  • In some gate drivers, such as TI’s Smart Gate Drivers, gate current can be easily configured through the IDRIVE setting without the need to redesign external circuitry between the gate drivers and external FETs. This provides designers more flexibility in configuring their system for EMI versus thermal tradeoffs. For more information on TI’s Smart Gate Drive technology, see Section 3.1.1.

Table 2-1 compares the specifications of gate driver and integrated FET driver architectures.