DRV10987 Datasheet, PDF (35/69 Pages) Texas Instruments – DRV10987 12- to 24-V, Three-Phase, Sensorless BLDC Motor Driver

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DRV10987
SLVSE89 – AUGUST 2017
8.4.8.1 Lock0: Lock-Detection Current Limit Triggered
The lock-detection current-limit function provides a configurable threshold for limiting the current to prevent
damage to the system. This is often tripped in the event of a sudden locked-rotor condition. The DRV10987
device continuously monitors the current in the low-side drivers as shown in Figure 30. If the current goes higher
than the threshold configured by the HWiLimitThr[2:0] bits, then the DRV10987 device stops driving the motor by
placing the output phases into a high-impedance state. The Lock0 bit is set and a lock condition is reported. The
device retries after t_{LOCK_OFF}.
Set the lock-detection current limit to a higher value than the acceleration current limit.
+
–
DAC
Digital Core
Figure 30. Lock-Detection Current Limit
8.4.8.2 Lock1: Abnormal Speed
If the motor is operating normally, the motor BEMF should always be less than the output amplitude. The
DRV10987 device uses two methods of monitoring the BEMF in the system. The U phase current is monitored to
maintain an estimate of BEMF based on the setting for Rm[6:0] {RmShift[2:0],RmValue[3:0]}. In addition, the
BEMF is estimated based on the operation speed of the motor and the setting for Kt[6:0]
{KtShift[2:0],KtValue[3:0]}. Figure 31 shows the method for using this information to detect a lock condition. If the
motor BEMF is much higher than the output amplitude for a certain period of time, t_{LCK_ETR}, it means the
estimated speed is wrong, and the motor has gotten out of phase.
Rm
I
BEMF1 = U – I × Rm
U
M
BEMF2 = Kt × speed
Lock detected if BEMF2 > U
Figure 31. Lock Detection 1
8.4.8.3 Lock2: Abnormal Kt
For any given motor, the integrated value of BEMF during half of an electrical cycle is constant. The value is
determined by the motor velocity constant (Kt_PH) (see Figure 32). The motor velocity constant is the same
regardless of whether the motor is running fast or slow. This constant value is continuously monitored by
calculation and used as a criterion to determine the motor lock condition, and is referred to as Ktc.
Based on the Kt_PH value programmed, create a range from Kt_low to Kt_high. If Ktc goes beyond the range for a
certain period of time, t_{LCK_ETR}, lock is detected. Kt_low and Kt_high are determined by KtLckThr[1:0] (see
Figure 33).
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