ST7MC1K2 datasheet(192/371 Pages) STMICROELECTRONICS

192 / 371 page

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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

hysteresis comparator referred to a selectable reference voltage.

IS[1:0] bits in MPHST register allow the input to be selected which drives to the comparator

(either MCIA, B or C). Be careful that the comparator is OFF until CKE and/or DAC bits are

set in MCRA register.

The VR[2:0] bits in the MCRC register select the reference voltage from seven internal

values depending on the noise level and the application voltage supply. The reference

voltage can also be set externally through the MCVREF pin when the VR[2:0] bits are set.

BEMF detections are performed during the measurement 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-

excited winding terminal, referred to the ground terminal.

The zero crossing sampling frequency is then defined, in current mode, by the

measurement window generator frequency (SA[3:0] bits in the MPRSR register) or, in

voltage mode, by the PWM generator frequency and phase U duty cycle.

During a short period after a phase commutation (C event), the winding where the back-emf

is read is no longer excited but needs a demagnetization 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 demagnetization event’ D must be detected on the winding before the detector can sense

a BEMF zero crossing.

For the end-of-demagnetization detection, no special PWM configuration is needed; the

comparator sensing is done at a selectable frequency (fSCF) (see Table 166).

So the three events C (commutation), D (demagnetization) and Z (BEMF zero crossing)

must always occur in this order in autoswitched mode when hard commutation is selected.

Table 83.

Threshold voltage setting

VR2

VR1

VR0

VREF voltage threshold

111

Threshold voltage set by external MCVREF pin

typical value for VDD =5V

3.5V

2.5V

100

1.5V

010

0.6V

0.2V

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