AN1946 Datasheet, PDF (25/35 Pages) STMicroelectronics – SENSORLESS BLDC MOTOR CONTROL AND BEMF SAMPLING METHODS WITH ST7MC

AN1946
6 MIXED SAMPLING AT END OF PWM OFF AND DURING PWM ON
Sampling at the end of PWM OFF limits the duty cycle and hence does not allow 100% of the
duty cycle on the motor.
Sampling during PWM ON needs a minimum ON state to be able to do sampling, so this can
be a limitation in case of a low load of the motor.
For both sampling methods there is a limitation.
We can use the features of ST7MC passing from one sampling method to the other, covering
all the PWM states.
The solution we present here proposes to start the motor using sampling at the end of PWM
OFF allowing the motor to be run with a short PWM On and at low speed.
The BEMF sampling at PWM OFF needs only a serial resistor between the motor winding and
the input voltage comparator of the microcontroller.
The sampling during PWM ON needs a voltage divider to get a voltage level compatible with
the input of the voltage comparator.
The software driver of a classical I/O of a microcontroller used to drive the switch to enable a
voltage divider on each input of the voltage comparator allows one configuration to pass to the
other without perturbation of signal when sampling is done at end of PWM "OFF".
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