TC646B View Datasheet(PDF) - Microchip Technology
Part Name
Description
Manufacturer
TC646B
Microchip Technology
TC646B Datasheet PDF : 36 Pages
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TC646B/TC648B/TC649B
4.0 DEVICE OPERATION
The TC646B/TC648B/TC649B devices are a family of
temperature-proportional, PWM mode, fan speed con-
trollers. Features of the family include minimum fan
speed, fan auto-shutdown, fan auto-restart, remote
shutdown, over-temperature indication and fan fault
detection.
The TC64XB 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 VOUT outputs no longer “latch” to a
state during a fan fault condition. The TC646B/
TC648B/TC649B family will continue to monitor the
operation of the fan so that when the fan returns to nor-
mal 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 a RMS current of 110 mA and an average cur-
rent of 72 mA. Using a MOSFET with a 1Ω RDS(on) (a
fairly typical value for this low current), the power dissi-
pation in the drive element would be: 12 mW (Irms2 *
RDS(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. This
allows smaller devices to be used and will not require
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 TC646B, TC648B and TC649B devices implement
PWM fan speed control by varying the duty cycle of a
fixed-frequency pulse train. The duty cycle of a wave-
form is the on time divided by the total period of the
pulse. For example, if we take a 100 Hz waveform
(10 ms) with an on time of 5.0 ms, the duty cycle of this
waveform is 50% (5.0 ms / 10.0 ms). This example is
shown in Figure 4-1.
t
ton
toff
D = Duty Cycle
D = ton / t
t = Period
t = 1/f
f = Frequency
FIGURE 4-1:
Waveform.
Duty Cycle of a PWM
The TC646B/TC648B/TC649B devices generate a
pulse train with a typical frequency of 30 Hz
(CF = 1 µF). The duty cycle can be varied from 0% to
100%. The pulse train generated by the TC646B/
TC648B/TC649B 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, “Output Drive
Device Selection”, for more information on output drive
device selection.
2003 Microchip Technology Inc.
DS21755B-page 11
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