DRV8461SPWPR datasheet(39/117 Pages) TI

39 / 117 page

Where UGB is the unity-gain bandwidth of the loop, RMOTOR is the motor coil resistance, LMOTOR is the motor coil

inductance, IFS is the full-scale current and VM is the supply voltage.

• If any frequency is less than UGB, it is allowed to pass.

• Frequencies higher than UGB, such as PWM frequency or STEP frequency are attenuated and do not

contribute to motor noise.

• 200 Hz is a reasonable choice for UGB to attenuate most frequencies in the audible range.

• In the event of supply voltage change, UGB can be changed by modifying the value of KP. This way similar

audio noise suppression can be achieved across a wide range of operating conditions.

• If the zero is chosen to be at a lower frequency than the motor pole, UGB will increase, as shown in the gain

vs frequency plot.

The zero should be placed to cancel the motor pole. By equating fP and fZ for a discretized implementation, the

following equation can be used to calculate KI.

KI = KP * RMOTOR / (FPWM * LMOTOR)

(10)

As an example, consider the following use case -

• VM = 24 V

• IFS = 3 A

• RMOTOR = 0.9 Ω

• LMOTOR = 1.2 mH

• UGB = 200 Hz

• FPWM = 25 kHz

• Above 50 RPM, the decay mode should change from silent step to smart tune ripple control.

Using the previous equations, KP = 0.1885 and KI = 0.00566. The following register values can be set -

• SS_KP = 0110000b = 48

• SS_KI = 0000011b = 3

• SS_KP_DIV_SEL = 011b = 1/256

• SS_KI_DIV_SEL = 100b = 1/512

• 50 RPM corresponds to roughly 42.6 kpps at 1/256 microstepping, which is equivalent to 42 Hz frequency of

the sinusoidal current waveform. So SS_THR = 00010101b = 21.

The following scopeshot shows the smooth sinusoidal coil current waveforms when the motor operates in silent

step decay mode.

Figure 7-19. Coil Current Waveform with Silent Step Decay

The SS_SMPL_SEL bits influence the smoothness of current waveform around zero crossing point. Default

value of 2 μs sampling time will work well for most motors and applications. In case current waveform distortion

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DRV8461

SLOSE98 – DECEMBER 2022

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