DRV8461SPWPR datasheet(61/117 Pages) TI

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Sometimes the automatic stall learning process might not be successful due to unstable torque count while the

motor is running or is stalled. For example, when the motor has high coil resistance or is running at very high or

low speeds, the torque count might vary a lot over time and the difference between steady count and stall count

might be small. In such cases, it is recommended not to use the automatic stall learning method. Instead, the

user should carefully study the steady count and torque count across the range of operating conditions and set

the threshold midway between the minimum steady count and the maximum stall count.

A stall threshold learnt at one speed may not work well for another speed. It is recommended to re-learn the stall

threshold every time the motor speed is changed by more than ± 10%.

Note

• The stall detection algorithm depends on back-EMF modifying the PWM off time. The back-EMF

is directly proportional to the speed of the motor. In order for stall detection to work reliably, motor

speed should be sufficiently high to be able to generate back-EMF of sufficient amplitude. Higher

motor coil resistance would require higher minimum speed for reliable stall detection.

• When the device goes from Disable mode (H-bridges Hi-z) to Active mode by toggling EN_OUT

bit or ENABLE pin; or when the device recovers from a fault by issuing a CLR_FLT, stall detection

fault may also be flagged. This is because of the time taken by TRQ_CNT to reach a value higher

than STL_TH. As a result of the stall fault, nFAULT may stay low (if STL_REP = 1b), and another

CLR_FLT will be needed to release stall fault and nFAULT pin. This can be prevented by -

– Enabling stall detection after enabling active mode (write EN_STALL = 1b only after writing

EN_OUT = 1b and making ENABLE = logic high)

– By starting the STEP pulses only after the bridge is in active mode or after a CLR_FLT

command has been issued to clear a fault condition.

• If there is loss of current regulation due to low supply voltage, high coil resistance or high speed of

the motor, stall detection may not work reliably, because the TRQ_COUNT can be erratic and may

jump to a high value. This can be checked and confirmed by looking at the coil current waveform.

If the coil current has a standard sinusoidal waveform, and the peak of the sinusoid reaches the

desired full-scale current, then stall detection will work reliably. If the current waveform is triangular

due to high speed or low supply voltage, stall detection algorithm might not work reliably.

• If EN_STL = 1b and auto-torque is also enabled, coil current goes to ATQ_TRQ_MAX when motor

stall is detected.

• If EN_STL = 0b and auto-torque is enabled, coil current goes to ATQ_TRQ_MIN when motor is

stalled.

7.3.19.6 Open-Load Detection (OL)

Open-load fault is detected -

• When the motor is running -

– If coil current drops below the open-load current threshold (IOL)

• When the motor is in a holding condition -

– If coil current drops below the ITRIP level set by the indexer

• If either of the above conditions persists for more than the open-load detection time (tOL)

When the device is operating with SPI interface, the EN_OL bit must be 1b to enable open-load detection.

The open-load detection time (tOL) is set as shown in Table 7-27 -

Table 7-27. Open-load Detection Time

Interface

OL_T

Maximum tOL (ms)

H/W interface

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DRV8461

SLOSE98 – DECEMBER 2022

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