AN1627 Freescale Semiconductor / Motorola, AN1627 Datasheet - Page 20

• Manufacturer Part Number: AN1627
• Description: Low Cost High Efficiency Sensorless Drive for Brushless dc Motor Using MC68HC (7)05MC4
• Manufacturer: Freescale Semiconductor / Motorola

6
6.1
6.1.1
An outline of the Back-EMF sensing circuit is shown below.
As explained in the theoretical part of this application note, the phase zero crossing event can be
detected at the moment when the branch voltage (of a free phase) crosses the half DC-bus voltage level.
The resistor network is used to divide sensed voltages down to a 0-15V voltage level. The comparators
sense the zero voltage difference of the input signal. The multiple resistors reduce the voltage across
each resistor component to an acceptable level. A simple RC (54k
comparators being disturbed by high voltage spikes produced by IGBT switching. The MUX selects the
phase comparator output, which corresponds to the current commutation stage. This Zero Crossing
Detection signal is transferred to the microcontroller’s Input Capture pin (TCAP2).
MOTOROLA
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+DC_Bus Phase A Phase B Phase C
HARDWARE
Power Stage Board
Back-EMF sensing
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Freescale Semiconductor, Inc.
Figure 6-1.
For More Information On This Product,
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Figure 6-2.
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Go to: www.freescale.com
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Back-EMF Sensing Circuit Diagram
The Zero Crossing Detection
MUX Command
MUX
× 1nF) filter prevents the
Zero Crossing
Detection signal
REV 0.2
AN1627