AN2009 datasheet(7/39 Pages) STMICROELECTRONICS

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

3 SENSORLESS CONTROL METHODS

In order to make the BEMF signal readable by the microcontroller and to detect the zero-

crossing voltage of this signal, there are two main methods which we will call the classic

method for the first one and the ST method for the second. The ST7FMC microcontroller is

suitable for both methods. Each method is declinable in sub-method and the ST7MC allows

complete flexibility on which sub-method to use. Please refer to the Application Note AN1946

for a complete view and details on all the methods as this document briefly covers only the 2

main methods.

3.1 CLASSIC METHOD

The classic method involves dividing and filtering the signal coming from the non-powered

phase to make it readable by the 5V microcontroller as shown in Figure 5. A virtual ground is

built and is used as the voltage reference to detect the zero-crossing event of the BEMF

signal. With this method, the PWM signal which is logically ANDed with the switch control

signal can be applied either on the high or on the low side switches as explained in the next

section. The ST7MC is able to detect the BEMF zero-crossing signal during either the PWM

signal ON time or OFF time.

AN2009 Datasheet(PDF) 7 Page - STMicroelectronics