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TC647B Scheda tecnica(PDF) 11 Page - Microchip Technology |
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TC647B Scheda tecnica(HTML) 11 Page - Microchip Technology |
11 / 36 page 2002-2013 Microchip Technology Inc. DS21756C-page 11 TC642B/TC647B 4.0 DEVICE OPERATION The TC642B/TC647B devices are a family of tempera- ture proportional, PWM mode, fan speed controllers. Features of the family include minimum fan speed, fan auto-shutdown mode, fan auto-restart, remote shut- down, over-temperature indication and fan fault detection. The TC642B/TC647B family is slightly different from the original TC64X family, which includes the TC642, TC646, TC647, TC648 and TC649 devices. Changes have been made to adjust the operation of the device during a fan fault condition. The key change to the TC64XB family of devices (TC642B, TC647B, TC646B, TC648B, TC649B) is that the FAULT and VOUT outputs no longer “latch” to a state during a fan fault condition. The TC64XB family will continue to monitor the operation of the fan so that when the fan returns to normal operation, the fan speed controller will also return to normal operation (PWM mode). The operation and features of these devices are discussed in the following sections. 4.1 Fan Speed Control Methods The speed of a DC brushless fan is proportional to the voltage across it. This relationship will vary from fan to fan and should be characterized on an individual basis. The speed versus applied voltage relationship can then be used to set up the fan speed control algorithm. There are two main methods for fan speed control. The first is pulse width modulation (PWM) and the second is linear. Using either method, the total system power requirement to run the fan is equal. The difference between the two methods is where the power is consumed. The following example compares the two methods for a 12V, 120 mA fan running at 50% speed. With 6V applied across the fan, the fan draws an average current of 68 mA. Using a linear control method, there is 6V across the fan and 6V across the drive element. With 6V and 68 mA, the drive element is dissipating 410 mW of power. Using the PWM approach, the fan voltage is modulated at a 50% duty cycle with most of the 12V being dropped across the fan. With 50% duty cycle, the fan draws an RMS current of 110 mA and an average current of 72 mA. Using a MOSFET with a 1 R DS(on) (a fairly typical value for this low current), the power dissipation in the drive element would be: 12 mW (Irms2 * RDS(on)). Using a standard 2N2222A NPN transistor (assuming a Vce-sat of 0.8V), the power dissipation would be 58 mW (Iavg* Vce-sat). The PWM approach to fan speed control results in much less power dissipation in the drive element, allowing smaller devices to be used while not requiring special heatsinking to remove the power being dissipated in the package. The other advantage of the PWM approach is that the voltage being applied to the fan is always near 12V. This eliminates any concern about not supplying a high enough voltage to run the internal fan components, which is very relevant in linear fan speed control. 4.2 PWM Fan Speed Control The TC642B and TC647B devices implement PWM fan speed control by varying the duty cycle of a fixed fre- quency pulse train. The duty cycle of a waveform is the on time divided by the total period of the pulse. For example, a 100 Hz waveform (10 ms) with an on time of 5.0 ms has a duty cycle of 50% (5.0 ms / 10.0 ms). This example is illustrated in Figure 4-1. FIGURE 4-1: Duty Cycle of a PWM Waveform. The TC642B and TC647B generate a pulse train with a typical frequency of 30 Hz (CF = 1 µF). The duty cycle can be varied from 0% to 100%. The pulse train gener- ated by the TC642B/TC647B device drives the gate of an external N-channel MOSFET or the base of an NPN transistor (shown in Figure 4-2). See Section 5.5, “Out- put Drive Device Selection”, for more information. FIGURE 4-2: PWM Fan Drive. t ton toff t = Period t = 1/f f = Frequency D = Duty Cycle D = ton / t FAN 12V QDRIVE TC642B TC647B VDD GND VOUT G D S |
Codice articolo simile - TC647B |
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Descrizione simile - TC647B |
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