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TC648VUA Datasheet(PDF) 6 Page - Microchip Technology |
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TC648VUA Datasheet(HTML) 6 Page - Microchip Technology |
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6 / 28 page  TC648 DS21448C-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 pin. A frequency of 30 Hz is recommended for most applications (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 approximately one second. 3.4 Over-Temperature Fault (OTF) Output OTF is asserted when the PWM control voltage applied to VIN becomes greater than that needed to drive 100% duty cycle (see Section 1.0, “Electrical Characteris- tics”). This indicates that the fan is at maximum drive, and the potential exists for system overheating. Either heat dissipation 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 obstruc- tion). This output may be treated as a “System Over- heat” warning and used to trigger system shutdown or some other corrective action. OTF will become inactive when VIN < VOTF. 3.5 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 TC648 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 VAS input should be grounded if auto- shutdown mode is not used. 3.6 Shutdown Mode (Reset) If an unconditional shutdown and/or device reset is desired, the TC648 may be placed in shutdown mode by forcing VIN to a logic low (i.e., VIN < VSHDN) (see Figure 3-1). In this mode, all functions cease and the OTF output is unconditionally inactive. The TC648 should not be shut down unless all heat producing activity in the system is at a negligible level. The TC648 exits shutdown mode when VIN becomes greater than VREL, the release voltage. Entering shutdown mode also performs a complete device reset. Shutdown mode resets the TC648 into its power-up state. OTF is unconditionally inactive in shut- down mode. Upon exiting shutdown mode (VIN > VREL), the Start-up Timer will be triggered and normal operation will resume, assuming VIN > VAS + VHAS If VIN is not greater than (VAS + VHAS) upon exiting shutdown mode, the fan will not be restarted. To ensure that a complete reset takes place, the user’s circuitry must ensure that VIN > (VAS + VHAS) when the device is released from shutdown mode. A recommended algorithm for management of the TC648 by a host microcontroller or other external circuitry is given in Section 5.0, “Typical Applications”. A small amount of hysteresis, typically one percent of VDD (50 mV at VDD = 5.0V), is designed into the VSHDN/VREL thresh- old. 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. Note: If VIN < VAS when the device exits shutdown mode, the fan will not restart as it will be in auto-shut- down mode. CAUTION: Shutdown mode is unconditional. That is, the fan will remain off as long as the VIN pin is being held low or VIN < VAS + VHAS. |
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