ST7MC1 datasheet(148/308 Pages) STMICROELECTRONICS

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ST7MC1/ST7MC2

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MOTOR CONTROLLER (Cont’d)

10.6.6.2 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.

IS[1:0] bits in MPHST register allow to select the

input which will be drive to the comparator (either

MCIA, B or C). Be careful that the comparator is

OFF until CKE and/or DAC bit are set in MCRA

The VR[2:0] bits in the MCRC register select the

reference voltage from seven internal values de-

pending on the noise level and the application volt-

age supply. The reference voltage can also be set

externally through the MCVREF pin when the

VR[2:0] bits are set.

Table 25. Threshold voltage setting

*Typical value for VDD=5V.

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 PWM

generator frequency and phase U duty cycle.

During a short period after a phase commutation

(C event), the winding where the back-emf will be

read is no longer excited but needs a demagneti-

sation 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 de-

tector 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 selectable frequency (fSCF),

see Table 82.

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

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

ways occur in this order in autoswitched mode

when hard commutation is selected.

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 30.

To avoid wrong detection of D and Z events, a

blanking window filter is implemented for spike fil-

tering. In addition, by means of an event counter,

software can filter several consecutive events up

to a programmed limit before generating the D or Z

event internally. This is shown in Figure 79 and

Figure 80.

VR2

VR1

VR0

Vref voltage threshold

111

Threshold voltage set by

external MCVREF pin

3.5V*

2.5V*

100

2V*

1.5V*

010

1V*

0.6V*

0.2V*

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