TC665TEUN View Datasheet(PDF) - Microchip Technology
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TC665TEUN Datasheet PDF : 36 Pages
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TC664/TC665
4.1 Fan Speed Control Methods
The speed of a DC brushless fan is proportional to the
voltage across it. For example, if a fan’s rating is
5000 RPM at 12 V, it’s speed would be 2500 RPM at
6 V. This, of course, will not be exact, but should be
close.
There are two main methods for fan speed control. The
first is pulse width modulation (PWM) and the second
is linear. Using either method the total system power
requirement to run the fan is equal. The difference
between the two methods is where the power is con-
sumed.
The following example compares the two methods for
a 12 V, 120 mA fan running at 50% speed. With 6 V
applied across the fan, the fan draws an average cur-
rent of 68 mA. Using a linear control method, there is
6V across the fan and 6V across the drive element.
With 6 V and 68 mA, the drive element is dissipating
410 mW of power. Using the PWM approach, the fan is
modulated at a 50% duty cycle, with most of the 12 V
being dropped across the fan. With 50% duty cycle, the
fan draws an RMS current of 110 mA and an average
current of 72 mA. Using a MOSFET with a 1 RDS(on)
(a fairly typical value for this low current) the power dis-
sipation in the drive element would be: 12 mW (Irms2 *
RDS(on)). Using a standard 2N2222A NPN transistor
(assuming a Vce-sat of 0.8 V), the power dissipation
would be 58 mW (Iavg* Vce-sat).
The PWM approach to fan speed control causes much
less power dissipation in the drive element. This allows
smaller devices to be used and will not require any spe-
cial heatsinking to get rid of the power being dissipated
in the package.
The other advantage to the PWM approach is that the
voltage being applied to the fan is always near 12 V.
This eliminates any concern about not supplying a high
enough voltage to run the internal fan components
which is very relevant in linear fan speed control.
4.2 PWM Fan Speed Control
The TC664/TC665 devices implement PWM fan speed
control by varying the duty cycle of a fixed frequency
pulse train. The duty cycle of a waveform is the on time
divided by the total period of the pulse. For example,
given a 100 Hz waveform (10 msec.) with an on time of
5.0 msec., the duty cycle of this waveform is 50%
(5.0 msec./10.0 msec.). An example of this is shown in
Figure 4-2.
T
Ton Toff
D = Duty Cycle
D = Ton / T
T = Period
T = 1/F
F = Frequency
FIGURE 4-2:
Waveform.
Duty Cycle Of A PWM
The TC664/TC665 devices generate a pulse train with
a typical frequency of 30 Hz (CF = 1 µF). The duty cycle
can be varied from 30% to 100%. The pulse train gen-
erated by the TC664/TC665 devices drives the gate of
an external N-channel MOSFET or the base of an NPN
transistor (Figure 4-3). See Section 7.5 for more infor-
mation on output drive device selection.
12 V
VDD
FAN
TC664 VOUT
TC665
GND
D
G Qdrive
S
FIGURE 4-3:
PWM Fan Drive.
By modulating the voltage applied to the gate of the
MOSFET Qdrive, the voltage applied to the fan is also
modulated. When the VOUT pulse is high, the gate of
the MOSFET is turned on, pulling the voltage at the
drain of Qdrive to zero volts. This places the full 12 V
across the fan for the Ton period of the pulse. When the
duty cycle of the drive pulse is 100% (full on, Ton = T),
the fan will run at full speed. As the duty cycle is
decreased (pulse on time “Ton” is lowered), the fan will
slow down proportionally. With the TC664/TC665
devices, the duty cycle can be controlled through the
analog input pin (VIN), or through the SMBus interface,
by using the Duty-Cycle Register. See Section 4.5 for
more details on duty cycle control.
DS21737B-page 10
2002-2013 Microchip Technology Inc.
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