Manually adjustable mixed decay schemes are not suitable for applications with variable step rates or variable loads. A variable mixed decay tuning ratio is required to accommodate these varying conditions. TI's solution to this problem is the smart tune technology, an integrated closed loop automatic decay tuning system. Smart tune automatically compensates for any changes with the motor operating parameters such as speed, load, voltage and temperature as well as aging effects.
There are two implementations of smart tune decay technology. Smart tune Dynamic Decay and Smart tune Ripple Control.
Smart tune Dynamic Decay: Within an electrical cycle of the winding current, namely increasing and decreasing current steps in both positive and negative quadrants, smart tune dynamic decay automatically selects between slow, mixed, and fast decay modes as shown in Figure 2-6. While mixed decay is selected, smart tune dynamically adjusts the fast decay percentage of the total mixed decay time thus eliminating the need for manual mixed decay tuning. The mixed decay percentages are evaluated and optimized during each PWM cycle of the winding current regulation for lowest achievable current ripple with a fixed off-time, tOFF, based regulation scheme. Smart tune dynamic decay is designed to operate with slow decay majority of the time and use as few as possible fast delay cycles necessary to maintain current regulation.
Smart tune Ripple Control: This method regulates the winding current with a preset current ripple value by using a variable off-time, tOFF, based regulation scheme. The value for the current ripple is defined by setting an IVALLEY value along with the ITRIP target current. When the current level reaches ITRIP, slow decay is initiated until IVALLEY is reached as shown in Figure 2-7. In this mode, tOFF is variable depending on the system operating parameters. Smart tune ripple control achieves low current ripple which in turn minimizes the audible noise from the motor.