TC647B Microchip Technology Inc., TC647B Datasheet - Page 20

• 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.

TC642B/TC647B
FIGURE 5-5:
5.5
The TC642B/TC647B is designed to drive an external
NPN transistor or N-channel MOSFET as the fan
speed modulating element. These two arrangements
are shown in Figure 5-7. For lower current fans, NPN
transistors are a very economical choice for the fan
drive device. It is recommended that, for higher current
fans (300 mA and above), MOSFETs be used as the
fan drive device. Table 5-2 provides some possible part
numbers for use as the fan drive element.
When using a NPN transistor as the fan drive element,
a base current-limiting resistor must be used, as is
shown in Figure 5-7.
When using MOSFETs as the fan drive element, it is
very easy to turn the MOSFETs on and off at very high
rates. Because the gate capacitances of these small
MOSFETs are very low, the TC642B/TC647B can
charge and discharge them very quickly, leading to
very fast edges. Of key concern is the turn-off edge of
the MOSFET. Since the fan motor winding is essentially
an inductor, once the MOSFET is turned off, the current
that was flowing through the motor wants to continue to
flow. If the fan does not have internal clamp diodes
around the windings of the motor, there is no path for
this current to flow through, and the voltage at the drain
of the MOSFET may rise until the drain-to-source rating
of the MOSFET is exceeded. This will most likely cause
the MOSFET to go into avalanche mode. Since there is
very little energy in this occurrence, it will probably not
fail the device, but it would be a long-term reliability
issue.
DS21756B-page 20
Output Drive Device Selection
Fan Current During a Locked Rotor Condition.
The following is recommended:
• Ask how the fan is designed. If the fan has clamp
FIGURE 5-6:
Off.
diodes internally, this problem will not be seen. If
the fan does not have internal clamp diodes, it is a
good idea to put one externally (Figure 5-6). Put-
ting a resistor between V
MOSFET will also help slow down the turn-off and
limit this condition.
V
OUT
Q
1
: N-Channel MOSFET
Clamp Diode For Fan Turn-
2003 Microchip Technology Inc.
OUT
FAN
GND
and the gate of the
R
Q
SENSE
1