The motor driver may experience different transient driving conditions that cause large currents to flow for a short duration of time. These may include

• Motor start-up when the rotor is initially stationary.
• Fault conditions when there is a supply or ground short to one of the motor outputs, and the overcurrent protection triggers.
• Briefly energizing a motor or solenoid for a limited time, then de-energizing.

For these transient cases, the duration of drive time is another factor that impacts thermal performance in addition to copper area and thickness. In transient cases, the thermal impedance parameter Z_θJA denotes the junction-to-ambient thermal performance. The figures in this section show the simulated thermal impedances for 1-oz and 2-oz copper layouts for the HSOP package. These graphs indicate better thermal performance with short current pulses. For short periods of drive time, the device die size and package dominates the thermal performance. For longer drive pulses, board layout has a more significant impact on thermal performance. Both graphs show the curves for thermal impedance split due to number of layers and copper area as the duration of the drive pulse duration increases. Long pulses can be considered steady-state performance.

Figure 9-21 HSOP package junction-to-ambient thermal impedance for 1-oz copper layouts

Figure 9-22 HSOP package junction-to-ambient thermal impedance for 2-oz copper layouts