TC642B Datasheet, PDF (11/34 Pages) Microchip Technology – PWM Fan Speed Controllers With Minimum Fan Speed, Fan Restart and FanSense™ Technology for Fault Detection

4.0 DEVICE OPERATION
The TC642B/TC647B devices are a family of tempera-
ture proportional, PWM mode, fan speed controllers.
Features of the family include minimum fan speed, fan
auto-shutdown mode, fan auto-restart, remote shut-
down, over-temperature indication and fan fault
detection.
The TC642B/TC647B family is slightly different from
the original TC64X family, which includes the TC642,
TC646, TC647, TC648 and TC649 devices. Changes
have been made to adjust the operation of the device
during a fan fault condition.
The key change to the TC64XB family of devices
(TC642B, TC647B, TC646B, TC648B, TC649B) is that
the FAULT and V_OUT outputs no longer “latch” to a
state during a fan fault condition. The TC64XB family
will continue to monitor the operation of the fan so that
when the fan returns to normal operation, the fan speed
controller will also return to normal operation (PWM
mode). The operation and features of these devices
are discussed in the following sections.
4.1 Fan Speed Control Methods
The speed of a DC brushless fan is proportional to the
voltage across it. This relationship will vary from fan to
fan and should be characterized on an individual basis.
The speed versus applied voltage relationship can then
be used to set up the fan speed control algorithm.
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
consumed.
The following example compares the two methods for
a 12V, 120 mA fan running at 50% speed. With 6V
applied across the fan, the fan draws an average
current of 68 mA.
Using a linear control method, there is 6V across the
fan and 6V across the drive element. With 6V and
68 mA, the drive element is dissipating 410 mW of
power.
Using the PWM approach, the fan voltage is modulated
at a 50% duty cycle with most of the 12V 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 ΩR_DS(on) (a fairly
typical value for this low current), the power dissipation
in the drive element would be: 12 mW (Irms² * R_DS(on)).
Using a standard 2N2222A NPN transistor (assuming
a Vce-sat of 0.8V), the power dissipation would be
58 mW (Iavg* Vce-sat).
The PWM approach to fan speed control results in
much less power dissipation in the drive element,
allowing smaller devices to be used while not requiring
TC642B/TC647B
special heatsinking to remove the power being
dissipated in the package.
The other advantage of the PWM approach is that the
voltage being applied to the fan is always near 12V.
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 TC642B and TC647B devices implement PWM fan
speed control by varying the duty cycle of a fixed fre-
quency pulse train. The duty cycle of a waveform is the
on time divided by the total period of the pulse. For
example, a 100 Hz waveform (10 ms) with an on time
of 5.0 ms has a duty cycle of 50% (5.0 ms / 10.0 ms).
This example is illustrated in Figure 4-1.
t
t_on
t_off
D = Duty Cycle
D = t_on / t
t = Period
t = 1/f
f = Frequency
FIGURE 4-1:
Waveform.
Duty Cycle of a PWM
The TC642B and TC647B generate a pulse train with a
typical frequency of 30 Hz (C_F = 1 µF). The duty cycle
can be varied from 0% to 100%. The pulse train gener-
ated by the TC642B/TC647B device drives the gate of
an external N-channel MOSFET or the base of an NPN
transistor (shown in Figure 4-2). See Section 5.5, “Out-
put Drive Device Selection”, for more information.
12V
V_DD
FAN
TC642B ^VOUT
TC647B
D
G
Q_DRIVE
S
GND
FIGURE 4-2:
PWM Fan Drive.
 2003 Microchip Technology Inc.
DS21756B-page 11