Increasing voltage slew rate results in voltage ringing (overshoots and undershoots) on the gate and source signals of the MOSFETs. The magnitude of the overshoots translates directly with the slew rate (controlled by gate current). See Figure 3-1 for the impact of gate current on slew rate and ringing. The responsibility falls on the system design engineer to ensure that the voltage spikes do not violate the absolute maximum ratings of the motor driver, specifically gate and source pin ratings. Operating above the ratings of the motor driver will affect device performance and reliability leading to unexpected behavior/rapid wear.

Figure 3-1 Impact of Gate Current on Slew Rate and Ringing : a) Gate Current - 64mA (left) b) Gate Current - 1024mA (right)

Another critical specification often overlooked is the absolute maximum slew rate tolerated by the source node of the motor driver. Many older devices and leading competitor devices have absolute maximum slew rate spec of 1V/us, limiting the switching speed. Also worth noting is that the MOSFET Voltage Drain-to-Source slew is not linear but rather resembles an S curve since this is a capacitor charging analog. This means that the slew rate can be higher than the device rating toward the middle portion. This poses a severe limitation on customers who need to explore faster slew switching times for switching efficiency. Texas Instruments' new 48V driver DRV8363-Q1 was designed specifically with this consideration in mind and offers a max 20V/ns slew rate tolerance on source node. DRV8363-Q1 new ratings increase system robustness to faster slew rate so that the driver does not become the limit for system performance.

Addition of RC snubbers or following proper PCB design practices can reduce the impact of ringing. Ultimately, the system design engineer must consider the EMI limitations and driver voltage ratings before selecting a gate current for the system. There are benefits and tradeoffs on either side including the overall PCB robustness to parasitics that impact the optimal slew rate for the system.