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TC646 Datasheet(PDF) 6 Page - Microchip Technology |
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TC646 Datasheet(HTML) 6 Page - Microchip Technology |
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6 / 28 page  TC646 DS21446C-page 6 2002 Microchip Technology Inc. 3.0 DETAILED DESCRIPTION 3.1 PWM The PWM circuit consists of a ramp generator and threshold detector. The frequency of the PWM is determined by the value of the capacitor connected to the CF input. A frequency of 30 Hz is recommended (CF = 1 µF). The PWM is also the time base for the Start-up Timer (see Section 3.3, “Start-Up Timer”). The PWM voltage control range is 1.25V to 2.65V (typical) for 0% to 100% output duty cycle. 3.2 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. 3.3 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 or auto- shutdown mode. If the PWM frequency is 30 Hz (CF = 1 µF), the resulting start-up time will be approxi- mately one second. If a fan fault is detected, the Diag- nostic Timer is triggered once, followed by the Start-up Timer. If the fault persists, the device is shut down (see Section 3.5, “FAULT Output”). 3.4 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 auto-shutdown or 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 differentiated 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, “Typical Applications”, for more details. 3.5 FAULT Output The TC646 detects faults in two ways: First, pulses appearing at SENSE due to the PWM turning on are blanked, with the remaining pulses being filtered by a missing pulse detector. If consecutive pulses are not detected for thirty-two PWM cycles ( ≅1Sec if CF = 1 µF), the Diagnostic Timer is activated and VOUT is driven high continuously for three PWM cycles ( ≅100 msec if CF = 1 µF). If a pulse is not detected within this window, the Start-up Timer is trig- gered (see Section 3.3, “Start-up Timer”). This should clear a transient fault condition. If the missing pulse detector times out again, the PWM is stopped and FAULT goes low. When FAULT is activated due to this condition, the device is latched in shutdown mode and will remain off indefinitely. Therefore, the TC646 is pre- vented from attempting to drive a fan under cata- strophic fault conditions. One of two things will restore operation: Cycling power off and then on again or pulling VIN below VSHDN and releasing it to a level above VREL. When one of these two conditions is satisfied, the normal start-up cycle is triggered and operation will resume if the fault has been cleared. The second condition by which the TC646 asserts 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 or some other corrective action. However, in this case, the fan will continue to run even when FAULT is asserted. If the system is allowed to continue operation, and the temperature (and thus VIN) falls, the FAULT output will become inactive when VIN < VOTF. 3.6 Auto-Shutdown Mode If the voltage on VIN becomes less than the voltage on VAS, the fan is automatically shut off (auto-shutdown mode). The TC646 exits auto-shutdown mode when the voltage on VIN becomes higher than the voltage on VAS by VHAS (the auto-shutdown Hysteresis Voltage (see Figure 3-1)). The Start-up Timer is triggered and normal operation is resumed upon exiting auto-shut- down mode. The FAULT output is unconditionally inactive in auto-shutdown mode. |
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