TC670 查看數據表（PDF） - Microchip Technology
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TC670 Datasheet PDF : 14 Pages
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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
be a worst-case rating and not the fan’s nominal oper-
ating current. 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
Nominal Fan Current
(mA)
RSENSE (Ω)
100
4.7
200
2.4
300
1.8
400
1.3
500
1.0
600
0.8
A 0.1 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 2.0, “Typical Performance
Curves”, the relationship between the threshold volt-
age 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.
TC670
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 cor-
rectly, 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 2.5 mA (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 sink current of this pin is 25 mA, it is
recommended that the current sinking into the ALERT
output does not exceed 20 mA.
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 as
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 to keep this pin away from
switching signals, such as the fan excitation signals in
order to avoid false signals on the SENSE pin.
2003 Microchip Technology Inc.
DS21688C-page 7
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