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TC647VPA データシート(PDF) 5 Page - Microchip Technology |
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TC647VPA データシート(HTML) 5 Page - Microchip Technology |
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5 / 28 page  2002 Microchip Technology Inc. DS21447C-page 5 TC647 3.0 DETAILED DESCRIPTION 3.1 PWM The PWM circuit consists of a ramp generator and threshold detector. The frequency of the PWM is deter- mined 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 an asymmetric complimentary 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 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 persists, the device is shut down (see Section 3.6, “FAULT Output”). 3.4 Shutdown Control (Optional) If VMIN (Pin 3) is pulled below VSHDN, the TC647 will go into shutdown mode. This can be accomplished by driving VMIN with an open-drain logic signal or using an external transistor, as shown in Figure 3-1. All functions 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/ VREL threshold. 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. 3.5 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 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. 3.6 FAULT Output 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 32 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 triggered (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. The TC647 may be configured to continuously attempt fan restarts, if so desired. 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. Note: At this point, action must be taken to restart the fan by momentarily pulling VMIN below VSHDN, or cycling system power. In either case, the fan cannot remain disabled due to a fault condition as severe system dam- age could result. If the fan cannot be restarted, the system should be shut down. |
同様の部品番号 - TC647VPA |
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同様の説明 - TC647VPA |
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