DRV8461SPWPR datasheet(38/117 Pages) TI

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Table 7-22. Silent step Parameter Table (continued)

Parameter

Description

SS_THR[7:0]

Programs the frequency at which the device transitions from silent step decay mode to another decay

mode programmed by the DECAY bits. This frequency corresponds to the frequency of the sinusoidal

current waveform.

•

00000001b = 2 Hz

•

00000010b = 4 Hz

•

11111111b = 510 Hz (default)

To convert the SS_THR threshold to STEP frequency (fSTEP) for a specified microstepping setting, Equation 6

should be used -

fSTEP = (SS_THR * 1000 * usm) / 256

(6)

Where usm corresponds to the number of microsteps (4, 16, 256 etc.). When the device is operating with custom

microstepping mode, use usm = 256 in Equation 6 when calculating the STEP frequency.

The gain vs frequency plot of the silent step loop is shown below -

Loop Gain (dB)

Frequency

Pole at Origin

UGB

Motor Pole

PI Zero

Attenuated Frequency

Allowed Frequency

UGB reduced

UGB increased

Figure 7-18. Silent step Gain vs. Frequency

The loop transfer function has two poles and one zero -

• One pole at origin

• One pole (fP) due to the motor coil resistance and inductance -

fP = RMOTOR / (2 * π * LMOTOR)

(7)

• One zero (fZ) created by the PI loop

fZ = (KI * FPWM) / (2 * π * KP)

(8)

The proportional gain KP should be chosen to achieve the desired loop gain. Use the following equation to

calculate the KP -

KP = 6 * π * UGB * LMOTOR / VM

(9)

DRV8461

SLOSE98 – DECEMBER 2022

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