Datasheet

 2002-2013 Microchip Technology Inc. DS21756C-page 13
TC642B/TC647B
FIGURE 4-4: V
IN
voltage vs. PWM duty
cycle(Typical).
The PWM duty cycle is also controlled by the V
MIN
pin
See Section 4.5, “Minimum Speed (V
MIN
Pin)”, for
more details on this function.
4.5 Minimum Speed (V
MIN
Pin)
For the TC642B and TC647B devices, pin 3 is the V
MIN
pin. This pin is used for setting the minimum fan speed
threshold.
The minimum fan speed function provides a way to set
a threshold for a minimum duty cycle on the V
OUT
out-
put. This in turn produces a minimum fan speed for the
user. The voltage range for the V
MIN
pin is the same as
that for the V
IN
pin (1.20V to 2.60V). The voltage at the
V
MIN
pin is set in this range so that as the voltage at the
V
IN
pin decreases below the V
MIN
voltage, the output
duty cycle will be controlled by the V
MIN
voltage. The
following equation can be used to determine the neces-
sary voltage at V
MIN
for a desired minimum duty cycle
on V
OUT
.
EQUATION V
MIN
VOLTAGE
Example: If a minimum duty cycle of 40% is desired,
the V
MIN
voltage should be set to:
EXAMPLE 4-1:
If the voltage at the V
IN
pin falls below 1.76V, the duty
cycle of the V
OUT
output will not decrease below the
40% value that is now set by the voltage at the V
MIN
pin. In this manner, the fan will continue to operate at
40% speed even when the temperature (voltage at V
IN
)
continues to decrease.
For the TC642B and TC647B devices, the V
MIN
pin is
also used as the shutdown pin. The V
SHDN
and V
REL
threshold voltages are characterized in the “Electrical
Characteristics” table of Section 1.0. If the V
MIN
pin
voltage is pulled below the V
SHDN
threshold, the device
will shut down (V
OUT
output goes to a low state, the
FAULT
pin is inactive). If the voltage on the V
MIN
pin
then rises above the release threshold (V
REL
), the
device will go through a Power-Up sequence. The
Power-Up sequence is shown later in Figure 4-9.
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 mode, the V
OUT
output
is actively held low. The output can be varied from 0%
duty cycle (full off) to 100% duty cycle (full on). As pre-
viously discussed, the duty cycle of the V
OUT
output is
controlled via the V
IN
input voltage along with the V
MIN
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. 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.
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 (%)
V
MIN
(V) = (DC * 1.4) + 1.20
DC = Desired Duty Cycle
100
V
MIN
(V) = (40 * 1.4) + 1.20
100
V
MIN
= 1.76V