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AXBM2001 Datasheet(PDF) 5 Page - Zetex Semiconductors |
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AXBM2001 Datasheet(HTML) 5 Page - Zetex Semiconductors |
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5 / 14 page  One problem with using an external potential divider (R1 & R2 in Figure 5) is that the motor becomes susceptible to supply voltage changes. Say the supply voltage falls, normally the speed would drop due to less voltage across the windings, however as the potential divider is across the supply, as the voltage falls so the voltage at the SPD pin will fall. As a lower SPD voltage represents a higher speed so a lowering of the supply therefore causes the fan to speed up. To overcome this it is probably best to use a Zener regulated supply to the ZXBM200x. The following Table 1 gives an indication of the values required for R1 and R2 against different thermistor values. It should be noted that these are only for guidance as the final value will depend upon the thermistor and motor characteristics and so will need fine adjustment by experimentation. If the ZXBM200x device is to be run from a Zener regulated supply to control the speed verses supply voltage variation so R1 and R2 will need allowances made for the voltage value of Zener chosen. As the Bipolar drivers have a slower turn-off characteristics when driven from the ZXCB200x series slightly different values of R2 are required over those for MOSFETs. When driving the SPD pin from a signal or thermistor remote from the fan or controller there may be a need to provide noise protection at the SPD pin. In order to maintain a smooth speed control characteristic this will take the form of a capacitor of between 100nF and 1 F connected between the SPD and Gnd pins somewhere close to the device. This should suffice for most purposes and can be determined for each particular application. A capacitor on this pin also has an effect on the start-up characteristic of the motor as the charging of the capacitor will lag behind the rise of the supply voltage. As a low voltage on the SPD pin represents a higher speed the effect at start-up, if the SPD value is not set for full speed, will be for the motor speed to increase past its allotted point before settling back to that required. Whilst for the capacitor values stated above this effect may not be noticeable, for values greater than 1 Fitcan be used to advantage. In situations where the motor will be regularly started at a speed lower than its practical start value it can help to get motors started, in effect giving the motor a ‘kick’ to start it. AN 36 - 5 Applications Note AN36 Issue1 - JULY 2002 Thermistor Value Supply Voltage MOSFET Bipolar R1 R2 R1 R2 4.7k 5V 3.3k 2.2k 3.3k 1.5k 12V 6.8k 1.2k 6.8k 1.0k 24V 15k 1.3k 15k 1.0k 10k 5V 5.6k 3.3k 5.6k 2.4k 12V 13k 2.2k 13k 1.8k 24V 36k 3.3k 33k 2.2k 47k 5V 24k 12k 22k 8.6k 12V 56k 9.1k 56k 7.5k 24V 130k 10k 130k 8.6k 100k 5V *** * 12V * * * * 24V * * * * Table 1 * Note: Not normally required - dependent upon thermistor and speed range needed. Figure 5 showing resistive divider used with <100k thermistors |
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