The start-up current setting is to control the peak start-up during open loop. During open-loop operation, it is desirable to control the magnitude of drive current applied to the motor. This is helpful in controlling and optimizing the rate of acceleration. The limit takes effect during reverse drive, align, and acceleration.

The start current is set by programming the OpenLCurr[1:0] bits. The current should be selected to allow the motor to reliably accelerate to the handoff threshold. Heavier loads may require a higher current setting, but it should be noted that the rate of acceleration is limited by the acceleration rate (StAccel[2:0], StAccel2[2:0]). If the motor is started with more current than necessary to reliably reach the handoff threshold, it results in higher power consumption.

The start current is controlled based on the relationship shown in Equation 4 and Figure 8-19. The duty cycle applied to the motor is derived from the calculated value for U_Limit and the magnitude of the supply voltage, V_CC, as well as the drive state of the motor.

Equation 4.

where

• I_Limit is configured by OpenLCurr[1:0]
• Rm is configured by Rm[6:0]
• Speed is variable based motor’s open loop acceleration profile
• Kt is configured by Kt[6:0]

Figure 8-19 Motor Start-Up Current