AN2687 Freescale Semiconductor / Motorola, AN2687 Datasheet - Page 6

• Manufacturer Part Number: AN2687
• Description: BLDC Fan Control using the MC68HC908QT2
• Manufacturer: Freescale Semiconductor / Motorola

AN2687/D
Power supply to the
MCU
Commutation and
Speed Control
Thermal Speed
Control
6
The basic operation is that the MCU will drive the transistors to let the BLDC
fan start and the Hall effect sensor will change the logic level according to the
position of the BLDC fan. Based on the signal from Hall effect sensor the MCU
controls the drive to the fan accordingly (commutation). The speed is
dependent on the input signal from the external thermal sensor. The MCU will
change the off-time period based on the input from this thermal sensor.
The detailed function of each block is described in the following paragraphs.
For component references, please refer to the schematic diagrams in
appendix A, at the back of this application note.
The power input to the BLDC fan is a regulated 12V, 1A supply to P1.
The Z1 is used in this reference design to act as a voltage regulator with 5%
accuracy to provide power to the MCU. The regulator is formed by R1, Z1, and
C1. The ratings and values of the components are dependent on the input
voltage, output voltage, output current, and output ripple requirements.
The commutation is achieved by using two MCU I/O pins. Pin-5 is configured
as the IRQ pin to sense the falling edge of the Hall signal. Pin-7 is configured
as the timer input capture to sense the rising edge of the Hall signal. Using the
IRQ interrupt provides faster response and smaller code size. The speed
control is based on the off-time period adjustment in both Q1 and Q2.
Figure 6
the BLDC fan. L1 (Q1, pin3) needs to synchronize with the falling edge of the
Hall signal, while L2 (Q2 pin3) needs to synchronize with the rising edge of the
Hall signal to ensure correct commutation. The fan speed is dependent on the
off-time period. For full speed of the BLDC fan, the off-time period equals to
zero. For lower speeds, the off-time period will increase and the on-time period
will decrease.
The speed of the fan is dependent on the temperature of the thermal sensor
that is connected to the MCU pin-6 in the evaluation demo or pin-2 in the
reference demo. Both pins are configured for ADC inputs to get the value from
the thermal sensor. The speed control algorithm adjusts the off-time period in
both Q1 and Q2 according to the thermal sensor reading. Since BLDC fan and
thermal sensor characteristics vary amongst different manufacturers, the
thermal speed profile needs to be tailored for different designs. A look-up table
for temperature-speed mapping provides an easy way to achieve this.
shows the actual temperature and speed relationship in this demo.
Freescale Semiconductor, Inc.
BLDC Fan Control using the MC68HC908QT2
For More Information On This Product,
shows the relationship between the commutation and speed control in
Go to: www.freescale.com
MOTOROLA
Figure 7