TC665 Microchip Technology Inc., TC665 Datasheet - Page 12

• Manufacturer Part Number: TC665
• Description: Smbus Pwm Fan Speed Controllers With Fan Fault Detection
• Manufacturer: Microchip Technology Inc.

TC664/TC665
FIGURE 4-6:
Network.
The second method for controlling the duty cycle of the
PWM output (V
to control the PWM duty cycle via the SMBus, DUTYC
(bit 5<0>) of the Configuration Register (Register 6.3)
must be set to a ‘1’. This tells the TC664/TC665 device
that the duty cycle should be controlled by the Duty
Cycle Register. Next, the Duty Cycle Register must be
programmed to the desired value. The Duty Cycle Reg-
ister is a 4 Bit read/write register that allows duty cycles
from 30% to 100% to be programmed. Table 4-1 shows
the binary codes for each possible duty cycle.
TABLE 4-1:
DS21737A-page 12
D(3)
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
34.8 k
14.7 k
D(2)
R
R
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
Duty-Cycle Register (Duty Cycle)
1
2
+5 V
OUT
D(1)
DUTY-CYCLE REGISTER
(DUTY-CYCLE) 4-BITS,
READ/WRITE
) is via the SMBus interface. In order
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
NTC Thermistor Sensor
C
0.01 µF
D(0)
NTC Thermistor
100 k
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
1
30%
34.67%
39.33% (default for V
open and when SMBus
is not selected)
44%
48.67%
53.33%
58%
62.67%
67.33%
72%
76.67%
81.33%
86%
90.67%
95.33%
100%
@ 25°C
GND
V
IN
Duty-Cycle
TC664
TC665
IN
This method of control allows for more sophisticated
algorithms to be implemented by utilizing microcontrol-
lers or microprocessors in the system. In this way, mul-
tiple system temperatures can be taken into account for
determining the necessary fan speed.
As shown in Table 4-1, the duty cycle has more of a
step function look than did the V
Because the step changes in duty cycle are small, they
are rarely audibly noticeable, especially when the fans
are integrated into the system.
4.6
The V
sistor or N-channel MOSFET as the low side switching
element in the system, as is shown in Figure 4-7. The
switching element is used to turn the fan on and off at
the PWM duty cycle commanded by the V
This output has complementary drive (pull up and pull
down) and is optimized for driving NPN transistors or
N-channel MOSFETs (see typical characteristic curves
for sink and source current capability of the V
stage).
The external device needs to be chosen to fit the volt-
age and current rating of the fan in a particular applica-
tion (Refer to Section 7.5 Output Drive Device
Selection). NPN transistors are often a good choice for
low current fans. If a NPN transistor is chosen, a base
current limiting resistor should be used. When using a
MOSFET as the switching element, it is sometimes a
good idea to have a gate resistor to help slow down the
turn on and turn off of the MOSFET. As with any switch-
ing waveform, fast rising and falling edges can
sometimes lead to noise problems.
As previously stated, the V
duty cycle during power up and release from shutdown
conditions. The V
commanded to do so via the Configuration Register
(SDM (bit 0<0>)). Even when a locked rotor condition
is detected, the V
the programmed duty cycle.
4.7
The TC664/TC665 devices also feature Microchip’s
proprietary FanSense technology. During normal fan
operation, commutation occurs as each pole of the fan
is energized. The fan current pulses created by the fan
commutation are sensed using a low value current
sense resistor in the ground return leg of the fan circuit.
The voltage pulses across the sense resistor are then
AC coupled through a capacitor to the SENSE pin of
the TC664/TC665 device. These pulses are utilized for
calculating the RPM of the fan. The threshold voltage
for the SENSE pin is 100 mV (typical). The peak of the
voltage pulse at the SENSE pin must exceed the
100 mV (typical) threshold in order for the pulse to be
counted in the fan RPM measurement.
OUT
PWM Output (V
Sensing Fan Operation (SENSE)
pin is designed to drive a low cost NPN tran-
OUT
OUT
output will continue to pulse at
output only shuts down when
2002 Microchip Technology Inc.
OUT
OUT
output will go to 100%
IN
)
control approach.
OUT
OUT
output
drive