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AXBM2001 Datasheet(PDF) 3 Page - Zetex Semiconductors |
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AXBM2001 Datasheet(HTML) 3 Page - Zetex Semiconductors |
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3 / 14 page  The buffered type of Hall sensor should be of the latching or bipolar type. These have an internal amplifier and thus provide a large amplitude square wave output. This signal is single ended and is applied into Pin 3, H+, as shown in Figure 2. On some buffered Hall sensors the output is of the Open Collector type and will therefore require a pull-up resistor (R1 in Figure 2) to attain the full signal amplitude. The H- pin will require to be held at a voltage approximately half the Hall output swing. The potential divider R2 and R3 in Figure 2 is provided for this. Speed Control Applied power, motor efficiency and mechanical loading determine maximum motor speed, the controller allows full speed adjustment by modifying the applied power only. Speed control is attained through an integral Pulse Width Modulation (PWM) circuit within the ZXBM200x series. The PWM signal controls the speed by switching the respective active Phase output at a much higher frequency, typically 25kHz, compared with the commutation 1 frequency. The PWM is used to control the percentage of time the output driver is turned on for. This PWM circuit is controlled through two pins, Pin 6, the CPWM pin, and Pin 4, the SPD input. The CPWM pin has a capacitor attached of 150pF to produce a 1V p-p 25kHz triangular waveform. The circuit has a lso been used quite successfully with 100pF to produce a PWM frequency of 34kHz. The motor speed is determined by the voltage at the SPD pin. There are two methods for controlling the ZXBM200x using the SPD pin. The first is the direct control of the SPD pin by a voltage from an external voltage source or system controller. The second method is to use a thermistor as a temperature sensor so as the ZXBM200x is in a feedback loop to control the fan speed against the temperature conditions. A third method of control is available by applying a PWM signal from an external source. All these will be discussed in more detail in the following sections. Voltage control of the SPD pin Voltage control is achieved by applying a voltage of between 1 volt and 2 volts to the SPD pin. A voltage of 1 volt on this pin will ensure the drivers are switched on for 100% of the PWM duty cycle i.e. the motor will run at full speed. Progressively increasing the SPD voltage to 2 volts will reduce the percentage of PWM duty cycle drive to the Phase windings to reduce the motor speed. With a SPD voltage of 2 volts the outputs will be switched off all the time i.e. no drive will be present. In reality a 2-phase DC brushless motor will have a minimum practical rotational speed below which the motor will not run. This is very much dependant upon the power, size and mechanics of the motor concerned but could well mean that anything less than 40% PWM drive, represented by a SPD voltage of 1.6V, is impractical. The ability of starting the motor under such low speed conditions also has to be considered as in most cases a motor will rotate at a far lower power input than is needed to start it. Further discussion will be given on this subject later. AN 36 - 3 Applications Note AN36 Issue1 - JULY 2002 Note: 1 Commutation is the process of alternate (phase) switching at the speed of rotation. |
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