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FAN3240 Scheda tecnica(PDF) 13 Page - ON Semiconductor |
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FAN3240 Scheda tecnica(HTML) 13 Page - ON Semiconductor |
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13 / 21 page  © 2011 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN3240 • Rev. 1.0.2 12 The inputs of the FAN324x are edge-sensitive inputs, which means that an input command is not recognized by the input being HIGH, but rather it is acknowledged when a LOW-to-HIGH transition is sensed at the input pin (IN1 or IN2). This is imperative to avoid erroneous output pulse generation in case an input would be permanently connected to a voltage above the TTL input threshold level. Such a scenario is plausible in case of a manufacturing mistake or if the signal source driving the INx pin stops working and stays permanently in HIGH state. a) Input Qualification One of the fundamental functions of the FAN324x is to qualify the incoming logic signals and differentiate between a valid command and noise at its inputs. For reliable operation, all input pulses shorter than the qualification time (tQUAL) of the device are ignored. That means that only qualified input pulses produce an energizing pulse for the relay at output of the device. An input signal becomes qualified if it stays continuously in the HIGH state for a time interval longer than the qualification time. Figure 21. Input Qualification Waveforms The waveforms shown in Figure 21 were taken with a FAN3240 device. The qualification time of this particular IC is 15 ms. The IN1 pulse width shown in Figure 21 is 10 ms long and it is ignored because it is shorter than tQUAL. The IN2 input receives a 50 ms wide pulse, which is longer than tQUAL; therefore, it qualifies as a valid command. Once the input signal stays continuously HIGH for longer than the qualification time, the relay drive output turns on (OUT2 waveform). In the FAN3240 device, the output pulse width equals the duration of the input signal, as shown in Figure 21. The length of the output pulse width (an exact copy of the incoming pulse width, as shown, or always the maximum limit), as well as the duration of the qualification time and the maximum allowable length of the output pulse, are factory-adjustable parameters and can be fine tuned to application requirements. These options and the adjustment range of these parameters are described in Section 6 of this Functional Description. The input qualification circuit provides additional protection against erroneous input pulses and noise at the input terminals. Figure 22 shows the presence of an erroneous input pulse at the IN2 input while the IN1 pulse is being qualified. Figure 22. Pending Qualification Waveform Before IN1 is fully qualified, an erroneous signal appears at the IN2 input. In this case, the qualification of IN1 continues, but the result is pending. If IN2 stays HIGH for longer than tQUAL, both inputs are valid. That scenario falls under XOR protection, which is discussed in the next section. In the example shown in Figure 22, IN2 is shorter than the qualification time and is ignored. At the time when IN2 goes LOW, the IN1 signal is already HIGH for longer than tQUAL. Therefore, IN1 is a valid command and is executed as demonstrated by the waveforms in Figure 22. b) Additional Input Protection Functions It is also possible that activating one of the inputs and its corresponding output signal will create noise in the system which will show up at the inactive input. Figure 23. Noise at the Inactive Input As Figure 23 shows, noise on the inactive input pin is suppressed and ignored. The FAN324x family of relay drivers can handle excessive noise signatures reliably. |
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