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AXBM2001 Datasheet(PDF) 6 Page - Zetex Semiconductors |
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AXBM2001 Datasheet(HTML) 6 Page - Zetex Semiconductors |
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6 / 14 page  External PWM The ZXBM200x series are also capable of being driven from an externally derived PWM signal. This is particularly useful where the device is to be used with existing system controllers that provide such an output. The PWM control waveform can be in the form of a digital signal. The main criteria being that it should have a low level of less than 1V and a high level greater than 2V (but not exceeding 5V). A conventional 3.3V or 5V based TTL or CMOS signal is ideal The signal is applied to Pin 8, the CPWM pin, with Pin 4, the SPD pin, being left open circuit. A low level on the CPWM pin represents the winding drive Off period and a high level the On period. Figure 6 illustrates a n external input waveform on the CPWM pin together with the resulting drive at one of the phase windings. There is some delay through the ZXBM200x device that ensures the drive reaches 100% PWM slightly before the input reaches 100% duty cycle. Fixed speed applications For applications where the speed control function is not required the device can be set to run at full speed by simply holding the SPD pin at a voltage below 1V. The best way to do this is by attaching a 10k resistor to Ground. A direct link to Ground can cause the SPD pin and its associated circuit not to function as required. Winding Drive The driving of the two phases of the motor is achieved using a pair of external power transistors driven from Pin 10 and Pin 9, Ph1 and Ph2 respectively, of the ZXBM200x controller. These transistors can be either of the Bipolar or MOSFET type. Both methodologies are similar and incorporate clamping to restrict the level of winding avalanche to be within the capabilities of the transistors chosen. Each Transistor type will be discussed in detail together with their respective merits and limitations. Bipolar Power Transistors Bipolar devices at one time were the universal power driving method, however, with MOSFETs becoming more readily available bipolar use has diminished. In the case of the ZXBM200x series it has been found that bipolar devices are very suited to the lower power, lower current end of the applications range. Figures 7 and 8 illustrate a bipolar driver output stage with two methods of clamping. In both Figures 7 and 8, R1 and R2 provide the base drive to the Output transistors Q1 and Q2. The value of these resistors should be chosen to provide sufficient current to maintain a good saturation voltage of the driver transistors but not to exceed the Ph1 and Ph2 drive capabilities of the ZXBM200x under worst case conditions. AN 36 - 6 Applications Note AN36 Issue1 - JULY 2002 Figure 6 |
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