TC654_14 Datasheet, PDF (12/38 Pages) Microchip Technology – Dual SMBus™ PWM Fan Speed Controllers With Fan Fault Detection

TC654/TC655
+5V
R₁
34.8 k
R₂
14.7 k
NTC Thermistor
100 k @ 25°C
V_IN
C₁
TC654/
0.01 µF
TC655
GND
FIGURE 4-6:
Network.
NTC Thermistor Sensor
The second method for controlling the duty cycle of the
PWM output (V_OUT) is via the SMBus interface. In order
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 TC654/TC655 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:
DUTY-CYCLE REGISTER
(DUTY-CYCLE) 4-BITS,
READ/WRITE
Duty-Cycle Register (Duty Cycle)
D(3) D(2) D(1) D(0)
Duty-Cycle
0
0
0
0 30%
0
0
0
1 34.67%
0
0
1
0 39.33% (default for V_IN
open and when SMBus
is not selected)
0
0
1
1 44%
0
1
0
0 48.67%
0
1
0
1 53.33%
0
1
1
0 58%
0
1
1
1 62.67%
1
0
0
0 67.33%
1
0
0
1 72%
1
0
1
0 76.67%
1
0
1
1 81.33%
1
1
0
0 86%
1
1
0
1 90.67%
1
1
1
0 95.33%
1
1
1
1 100%
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_IN control approach.
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 PWM Output (V_OUT)
The V_OUT pin is designed to drive two low-cost NPN
transistors or N-channel MOSFETs as the low side
power switching elements in the system as is shown in
Figure 4-7. These switching elements are used to turn
the fans on and off at the PWM duty cycle commanded
by the V_OUT output.
This output has complementary drive (pull up and pull
down) and is optimized for driving NPN transistors or
N-channel MOSFETs (see Section 2.0 “Typical Per-
formance CURVES” for sink and source current capa-
bility of the V_OUT drive 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” Output Drive Device Selection). NPN tran-
sistors are often a good choice for low-current fans. If a
NPN transistor is chosen, a base current-limiting resis-
tor 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 switching waveform,
fast rising and falling edges can sometimes lead to
noise problems.
As previously stated, the V_OUT output will go to 100%
duty cycle during power-up and release from shutdown
conditions. The V_OUT output only shuts down when
commanded to do so via the Configuration Register
(SDM (bit 0<0>)). Even when a locked rotor condition
is detected, the V_OUT output will continue to pulse at
the programmed duty cycle.
4.7 Sensing Fan Operation (SENSE1 &
SENSE2)
The TC654 and TC655 also feature Microchip's propri-
etary FanSense technology. During normal fan opera-
tion, commutation occurs as each pole of the fan is
energized. The fan current pulses created by the fan
commutation are sensed using low value current sense
resistors in the ground return leg of the fan circuit. The
voltage pulses across the sense resistor are then AC
coupled through capacitors to the SENSE pins of the
TC654/TC655 device. These pulses are utilized for cal-
culating the RPM of the individual fans. The threshold
voltage for the SENSE pins is 100 mV (typical). The
DS20001734C-page 12
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