ST72141K2 datasheet(46/132 Pages) STMICROELECTRONICS

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ST72141K

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MOTOR CONTROLLER (Cont’d)

Sensorless Mode

This mode is used to detect BEMF zero crossing

and end of demagnetization events.

The analog phase multiplexer connects the non-

excited motor winding to an analog 100mV hyster-

esis comparator referred to a selectable reference

voltage.

The VR[1:0] bits in the MCRB register select the

reference voltage from four internal values de-

pending on the noise level and the application volt-

age supply.

BEMF detections are performed during the meas-

urement window, when the excited windings are

free-wheeling through the low side switches and

diodes. At this stage the common star connection

voltage is near to ground voltage (instead of VDD/2

when the excited windings are powered) and the

complete BEMF voltage is present on the non-ex-

cited winding terminal, referred to the ground ter-

minal.

The zero crossing sampling frequency is then de-

fined, in current mode, by the measurement win-

dow generator frequency (SA[3:0] bits in the

MPRSR register) or, in voltage mode, by the 16-bit

Timer PWM frequency and duty cycle.

During a short period after a phase commutation

(C event), the winding is no longer excited but

needs a demagnetisation phase during which the

BEMF cannot be read. A demagnetization current

goes through the free-wheeling diodes and the

winding voltage is stuck at the high voltage or to

the ground terminal. For this reason an “end of de-

magnetization event” D must be detected on the

winding before the detector can sense a BEMF

zero crossing.

For the end-of-demagnetization detection, no spe-

cial PWM configuration is needed, the comparator

sensing is done at a fixed 800kHz sampling fre-

quency.

So, the three events: C (commutation), D (demag-

netization) and Z (BEMF zero crossing) must al-

ways occur in this order.

The comparator output is processed by a detector

that automatically recognizes the D or Z event, de-

pending on the CPB or ZVD edge and level config-

uration bits as described in Table 12.

A 20

−µs filter after a C event disables a D event if

spurious spikes occur.

Another 20

−µs filter after a D event disables a Z

event if spurious spikes occur.

Table 12. ZVD and CPB Edge Selection Bits

ZVD bit

CPB bit

Event generation vs input data sampled

Note: The ZVD bit is located in the MPAR register, the CPB bit is in the MCRB register.

CDH

20-µs filter

CDH

20-µs filter

CDH

20-µs filter

CDH

20-µs filter

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