AN2009 datasheet(12/39 Pages) STMICROELECTRONICS

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

4.1 PWM MANAGER IN VOLTAGE MODE

4.1.1 Description

Figure 8. Current limitation in voltage mode control

In voltage mode, the PWM of the 12-bit timer (through the compare U register) gives the

voltage which is supplied to the motor, it is the voltage control of the motor. This PWM signal

is the one logically ANDed with the control switches signal in order to be able to detect the

zero-crossing and demagnetization events.

In voltage control mode, we can set a limitation to the current. The current limitation can be set

by the user with an external resistor divider as shown in Figure 7 (R1 and R2) and Figure 8

(Rext). Usually this current limitation is the one given by the motor manufacturer. When the

current feedback reaches the maximum reference current at the comparator input, the tran-

sistor to which the PWM is applied is put in off state until the current feedback becomes less

than the maximum current limit. So, one of the inputs of the internal comparator is the max-

imum current limitation, the other input is the current feedback from the motor (MCCFI0 pin or

the output of the internal operational amplifier if used as shown in Figure 7).

This current limitation is for protection and should normally never be reached when running

the motor correctly.

Figure 9 is a capture made with an oscilloscope of the different signals in Voltage mode:

We can see that the current feedback from the motor never reaches the current limitation and

the increase and decrease of the current in the motor corresponds to the PWM signal on the

waveform 2.

set

TIMER

ST7MC

Microcontroller

Motor

Voltage

step time

T1-T4

T1-T6

ST7MC

PWM U output

Vdd

VR02139M

Max current

reference

Rext

Max current

reference

- +

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