March 2007

Rev 1

1/9

AN2502

Application note

Enhanced sensorless startup control of BLDC motors

using ST7FMC

Introduction

The ST7FMC microcontroller is capable of controlling 3 phase brushless DC (BLDC) motors

in position sensorless mode because of its powerful motor control macro cell. It is capable of

monitoring zero crossings of all 3 back EMF (BEMF) voltages using a direct BEMF detection

feature, where the entire motor terminal voltage is directly applied to an onchip zero

crossing detector during the zero crossing period without any attenuation. Hence it is able to

track the BEMF zero crossings even at very low motor speeds.

This direct BEMF detection feature helps to start the motor in auto commutated mode right

from zero speed. In traditional methods, where the motor voltage signals are scaled down at

the comparator input, it is necessary to step the rotor through a few rotations initially to let

the comparator see sufficient levels of BEMF signals to detect zero crossings. The rate at

which the rotor can be stepped through these initial rotations is highly influenced by load

inertia, and hence a step timing table has to be created on the basis of load inertia. This can

be a cumbersome task if the load inertia is not constant. With ST7FMC, because of the

direct BEMF detection feature, this procedure can be avoided completely, thereby saving on

development time as well as code memory space.

In any case, an alignment of the rotor to a known position is mandatory. At the end of

alignment, if a commutation is performed by switching the next set of windings into

conduction, the rotor is forced to turn in the direction of new step. If the winding current is

set close to rated motor current at the end of alignment, then this step change will apply a

rated torque on the rotor. Generally, this torque should be sufficient to kick the rotor to turn in

the right direction at a reasonable rate (or speed), inducing a reasonable BEMF in the

floating winding good enough for the zero crossing detector to identify a zero crossing as the

full BEMF is available at the comparator input without any attenuation. Hence from the very

first step, the device can be set in auto commutated mode like in a Hall position sensor

based control.

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