Datasheet

 2002-2013 Microchip Technology Inc. DS21755C-page 13
TC646B/TC648B/TC649B
For the TC646B, TC648B and TC649B devices, the V
IN
pin is also used as the shutdown pin. The V
SHDN
and
V
REL
threshold voltages are characterized in the “Elec-
trical Characteristics Table” of Section 1.0. If the V
IN
pin
voltage is pulled below the V
SHDN
threshold, the device
will shut down (V
OUT
output goes to a low state, the
FAULT
/OTF pin is inactive). If the voltage on the V
IN
pin
then rises above the release threshold (V
REL
), the
device will go through a power-up sequence (assuming
that the V
IN
voltage is also higher than the voltage at
the V
AS
pin). The power-up sequence is shown later in
the “Behavioral Algorithm Flowcharts” of Section 4.9.
FIGURE 4-4: V
IN
Voltage vs. PWM Duty
Cycle (Typical).
4.5 Auto-Shutdown Mode (V
AS
)
For the TC646B, TC648B and TC649B devices, pin 3
is the V
AS
pin and is used for setting the auto-shutdown
threshold voltage.
The auto-shutdown function provides a way to set a
threshold voltage (temperature) at which the fan will be
shut off. This way, if the temperature in the system
reaches a threshold at which the fan(s) no longer needs
to operate, the fan can be shutdown automatically.
The voltage range for the V
AS
pin is the same as the
voltage range for the V
IN
pin (1.20V to 2.60V). The volt-
age at the V
AS
pin is set in this range so that when the
voltage at the V
IN
pin decreases below the voltage at
the V
AS
pin (signifying that the threshold temperature
has been reached), the V
OUT
output is shut off (goes to
a low state). In auto-shutdown, the FAULT
/OTF output
is inactive (high-impedance). Auto-shutdown mode is
exited when the V
IN
voltage exceeds the V
AS
voltage
by the auto-shutdown hysteresis voltage (V
HAS
). Upon
exiting auto-shutdown mode, the start-up timer is
triggered and the device returns to normal operation.
4.6 V
OUT
Output (PWM Output)
The V
OUT
output is a digital output designed for driving
the base of a transistor or the gate of a MOSFET. The
V
OUT
output is designed to be able to quickly raise the
base current or the gate voltage of the external drive
device to its final value.
When the device is in shutdown/auto-shutdown mode,
the V
OUT
output is actively held low. The output can be
varied from 0% (full off) to 100% duty cycle (full on). As
previously discussed, the duty cycle of the V
OUT
output
is controlled via the V
IN
input voltage and can be termi-
nated based on the V
AS
voltage.
A base current-limiting resistor is required when using
a transistor as the external drive device in order to limit
the amount of drive current that is drawn from the V
OUT
output.
The V
OUT
output can be directly connected to the gate
of an external MOSFET. One concern when doing this,
though, is that the fast turn-off time of the fan drive
MOSFET can cause a problem because the fan motor
looks like an inductor. When the MOSFET is turned off
quickly, the current in the fan wants to continue to flow
in the same direction. This causes the voltage at the
drain of the MOSFET to rise. If there aren’t any clamp
diodes internal to the fan, this voltage can rise above
the drain-to-source voltage rating of the MOSFET. For
this reason, an external clamp diode is suggested. This
is shown in Figure 4-5.
FIGURE 4-5: Clamp Diode for Fan.
4.7 FAULT/OTF Output
The FAULT/OTF output is an open-drain, active-low
output. For the TC646B and TC649B devices, pin 6 is
labeled as the FAULT
output and indicates when a fan
fault condition has occurred. For the TC646B device,
the FAULT
output also indicates when an over-temper-
ature (OTF) condition has occurred. For the TC648B
device, pin 6 is the OTF
output that indicates an over-
temperature (OTF) condition has occurred.
0
10
20
30
40
50
60
70
80
90
100
1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8
V
IN
(V)
Duty Cycle (%)
Q
1
GND
R
SENSE
V
OUT
Q
1
: N-Channel MOSFET
FAN
Clamp Diode