AN2009 datasheet(19/39 Pages) STMICROELECTRONICS

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PWM MANAGEMENT FOR 3-PHASE BLDC MOTOR DRIVES USING THE ST7MC

5 MANAGEMENT OF PWM AND READING OF BEMF

Depending on the control method used to drive the motor, the PWM signal to detect the BEMF

zero voltage crossing signal can be applied in 2 ways:

– On the high side: During the step, the PWM is applied only on the high side switch, the low

side is ON during the complete step.

– On the low side: During the step, the PWM is applied only on the low side switch, the high

side is ON during the complete step.

In the ST7MC device, OE [5:0] bits in the MPAR register are used to define whether the cor-

responding outputs are in high or low side switch position. Then OO [5:0] bits in MPHST reg-

ister define if the channel is active or not and finally OS [2:0] bits in MCRB register distribute

the PWM on the active channels high or low side. Please refer to the product datasheet for the

motor control device in the channel manager section for more details or to Section 6 to see a

complete configuration.

Figure 12. Current during PWM ON time

5.1 PWM ON THE HIGH SIDE

When the related switches are ON, the current passes through the two switches and two

motor windings (these are switches T2 and T3 in the example in Figure 12).

The potential at point M is HV/2. If nothing is done, the BEMF can not be read by the ST7MC

because the voltage is too high to be read by the microcontroller.

Using the classic method, the BEMF is divided and filtered as is the rebuilt virtual ground and

so the BEMF zero-crossing event can be read when the PWM is ON on the high side switch.

The 2 switches T2 and T3 are ON

AN2009 Datasheet(PDF) 19 Page - STMicroelectronics