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TC642DEMO Datasheet(PDF) 6 Page - Microchip Technology |
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TC642DEMO Datasheet(HTML) 6 Page - Microchip Technology |
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6 / 28 page  TC642 DS21444C-page 6 2002 Microchip Technology Inc. The second condition by which the TC642 detects a fault is when the PWM control voltage applied to VIN becomes greater than that needed to drive 100% duty cycle (see Section 1.0, “Electrical Characteristics”). This indicates that the fan is at maximum drive and the potential exists for system overheating. Either heat dis- sipation in the system has gone beyond the cooling system’s design limits or some subtle fault exists (such as fan bearing failure or an airflow obstruction). This output may be treated as a system overheat warning and be used to trigger system shutdown. However, in this case, the fan will continue to run even when FAULT is asserted. If a shutdown is desired, FAULT may be connected to VMIN outside the device. This will latch the TC642 in shutdown mode when any fault occurs. 3.3 VOUT Output The VOUT pin is designed to drive a low cost transistor or MOSFET as the low side power switching element in the system. Various examples of driver circuits will be shown throughout this data sheet. This output has asymmetric complementary drive and is optimized for driving NPN transistors or N-channel MOSFETs. Since the system relies on PWM rather than linear control, the power dissipation in the power switch is kept to a minimum. Generally, very small devices (TO-92 or SOT packages) will suffice. 3.4 Start-Up Timer To ensure reliable fan start-up, the Start-up Timer turns the VOUT output on for 32 cycles of the PWM whenever the fan is started from the off state. This occurs at power-up and when coming out of shutdown mode. If the PWM frequency is 30 Hz (CF = 1 µF), the resulting start-up time will be approximately one second. If a fault is detected, the Diagnostic Timer is triggered once, followed by the Start-up Timer. If the fault per- sists, the device is shut down (see Section 3.2, “FAULT Output”). 3.5 Shutdown Control (Optional) If VMIN (Pin 3) is pulled below VSHDN, the TC642 will go into shutdown mode. This can be accomplished by driving VMIN with an open-drain logic signal or by using an external transistor, as shown in Figure 3-1. All func- tions are suspended until the voltage on VMIN becomes higher than VREL (0.85V @ VDD = 5.0V). Pulling VMIN below VSHDN will always result in complete device shutdown and reset. The FAULT output is unconditionally inactive in shutdown mode. A small amount of hysteresis, typically one percent of VDD (50 mV at VDD = 5.0V), is designed into the VSHDN and VREL thresholds. The levels specified for VSHDN and VREL in Section 1.0, “Electrical Characteristics”, include this hysteresis, plus adequate margin to account for normal variations in the absolute value of the threshold and hysteresis. 3.6 SENSE Input (FanSense Technology) The SENSE input (Pin 5) is connected to a low value current sensing resistor in the ground return leg of the fan circuit. During normal fan operation, commutation occurs as each pole of the fan is energized. This causes brief interruptions in the fan current, seen as pulses across the sense resistor. If the device is not in shutdown mode, and pulses are not appearing at the SENSE input, a fault exists. The short, rapid change in fan current (high dI/dt) causes a corresponding dV/dt across the sense resistor, RSENSE. The waveform on RSENSE is differen- tiated and converted to a logic-level pulse-train by CSENSE and the internal signal processing circuitry. The presence and frequency of this pulse-train is a direct indication of fan operation (see Section 5.0, “Typ- ical Applications”, for more details). CAUTION: Shutdown mode is unconditional. That is, the fan will not be activated regardless of the voltage at VIN. The fan should not be shut down until all heat producing activity in the system is at a negligible level. |
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