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TC670 Datasheet(PDF) 6 Page - Microchip Technology |
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TC670 Datasheet(HTML) 6 Page - Microchip Technology |
6 / 12 page TC670 DS21688B-page 6 © 2002 Microchip Technology Inc. The design of the proper input SENSE circuitry is a matter of scaling RSENSE to provide the necessary amount of gain and proper selection of the sensing capacitor. The following table (Table 4-1) lists some recommended values for RSENSE according to the nominal operating current of the fan. Please note that the current draw specified by the fan manufacturer may not be the fan’s nominal operating current, but a worst- case rating. If the fan current falls between two of the values listed, it is recommended that the higher value resistor is used. TABLE 4-1: RECOMMENDED VALUES FOR RSENSE PER FIGURE 4-2 A0.1 µF ceramic capacitor is recommended for CSENSE. Smaller capacitor values will require larger sense resistors whereas larger capacitors are more expensive and occupy more board space. 4.2 THRESHOLD Input The voltage at the THRESHOLD input sets the equivalent minimum allowable fan speed for the appli- cation. As shown in Section 3.0 typical performance curves, the relationship between the THRESHOLD voltage and minimum fan speed is also power supply and temperature dependant. All the values for the THRESHOLD voltage that are shown in these graphs represent typical numbers and might not be optimized for all fans in all applications. To ensure accurate fan speed monitoring of a specific fan in a specific application, the user must perform a one- time correlation check with the prototype. There are two techniques that can be used to calibrate the system. One approach is to find the fan’s full scale capability and mathematically estimate the minimum acceptable speed of the fan. A second technique is to identify the fan’s minimum speed and calibrate the THRESHOLD voltage accordingly. 4.2.1 THRESHOLD CALIBRATION USING FAN’S FULL SCALE SPEED The fan should first be run at full speed. At full speed the THRESHOLD voltage level should be adjusted until the ALERT output is asserted. With this full scale value of the THRESHOLD voltage, the value can be scaled down to the Fan Fault speed as a percentage of the full speed. For example, if the fan full speed THRESHOLD voltage is 1.5V, then the Fan Fault THRESHOLD voltage at 30% of full speed would be 30% x 1.5V = 0.45V. 4.2.2 THRESHOLD CALIBRATION USING FAN’S MINIMUM ALLOWABLE SPEED ESTIMATE For a more exact Fan Fault trip point, the user can run the fan at its minimum allowed speed. At this speed, the THRESHOLD voltage can be adjusted until the ALERT output is asserted. 4.3 CLEAR Input The CLEAR input allows the user to reset the ALERT pin to a high status. This is an active HIGH input. Consequently, as long as CLEAR is HIGH, ALERT will always be HIGH as well. To allow ALERT to operate correctly CLEAR must be held LOW. This feature can be implemented so that false Fan Fault conditions do not initiate system shutdown. 4.4 ALERT Output The ALERT output is an open drain output capable of sinking 2.5mA (typ). The ALERT output is asserted whenever the detected fan speed equals or falls below the equivalent voltage set at the THRESHOLD pin. The ALERT output is only deactivated once the CLEAR pin is brought to a HIGH state. Although the absolute maximum output current of this pin is 25mA, it is recommended that this current sinking into the ALERT Output does not exceed 20mA. 4.5 Power Supply Input, VDD To assure proper operation of the TC670 in a noisy environment where the fans are running, the VDD pin (Pin 4) must be decoupled with a 0.1 µF capacitor as shown in Figure 4-1. This capacitor should be located as close to the TC670 VDD pin as possible as well being promptly terminated to the ground plane. A Ceramic capacitor is recommended. 4.6 Ground Terminal, GND The GND pin (Pin 2) of the TC670 should be connected directly to the analog ground plane of the circuit board. Care should be taken in circuit layout to keep this pin away from switching signals, such as the fan excitation signals in order to avoid false signals on the SENSE pin. Nominal Fan Current (mA) RSENSE (Ω) 100 4.7 200 2.4 300 1.8 400 1.3 500 1.0 600 0.8 |
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