TC647B Microchip Technology Inc., TC647B Datasheet - Page 13

• Manufacturer Part Number: TC647B
• Description: Pwm Fan Speed Controllers With Minimum Fan Speed, Fan Restart And Fansense Technology For Fault Detection
• Manufacturer: Microchip Technology Inc.

FIGURE 4-4:
cycle(Typical).
The PWM duty cycle is also controlled by the V
See Section 4.5, “Minimum Speed (V
more details on this function.
4.5
For the TC642B and TC647B devices, pin 3 is the V
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
put. This in turn produces a minimum fan speed for the
user. The voltage range for the V
that for the V
V
V
duty cycle will be controlled by the V
following equation can be used to determine the neces-
sary voltage at V
on V
EQUATION
Example: If a minimum duty cycle of 40% is desired,
the V
EXAMPLE 4-1:
MIN
IN
DC = Desired Duty Cycle
2003 Microchip Technology Inc.
pin decreases below the V
100
OUT
pin is set in this range so that as the voltage at the
MIN
90
80
70
60
50
40
30
20
10
0
1
.
Minimum Speed (V
voltage should be set to:
1.2
IN
V
V
MIN
pin (1.20V to 2.60V). The voltage at the
MIN
1.4
MIN
(V) = (DC * 1.4) + 1.20
(V) = (40 * 1.4) + 1.20
V
for a desired minimum duty cycle
1.6
MIN
V
V
IN
MIN
1.8
= 1.76V
V
100
100
voltage vs. PWM duty
IN
VOLTAGE
(V)
2
MIN
MIN
MIN
2.2
voltage, the output
Pin)
pin is the same as
MIN
2.4
MIN
voltage. The
2.6
Pin)”, for
OUT
MIN
2.8
out-
MIN
pin
If the voltage at the V
cycle of the V
40% value that is now set by the voltage at the V
pin. In this manner, the fan will continue to operate at
40% speed even when the temperature (voltage at V
continues to decrease.
For the TC642B and TC647B devices, the V
also used as the shutdown pin. The V
threshold voltages are characterized in the “Electrical
Characteristics” table of Section 1.0. If the V
voltage is pulled below the V
will shut down (V
FAULT pin is inactive). If the voltage on the V
then rises above the release threshold (V
device will go through a Power-Up sequence. The
Power-Up sequence is shown later in Figure 4-9.
4.6
The V
the base of a transistor or the gate of a MOSFET. The
V
base current or the gate voltage of the external drive
device to its final value.
When the device is in shutdown mode, the V
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
controlled via the V
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
output.
The V
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.
OUT
output is designed to be able to quickly raise the
OUT
OUT
V
output is a digital output designed for driving
output can be directly connected to the gate
OUT
TC642B/TC647B
OUT
Output (PWM Output)
OUT
output will not decrease below the
IN
input voltage along with the V
IN
output goes to a low state, the
pin falls below 1.76V, the duty
SHDN
threshold, the device
DS21756B-page 13
SHDN
OUT
OUT
MIN
and V
REL
output is
MIN
MIN
output
), the
pin is
OUT
REL
MIN
MIN
pin
pin
IN
)