TC64X/TC64XB Fan Control Demo Board User’s Guide
DS21401C-page 6 2003 Microchip Technology Inc.
2.1.2 Using Logic Level MOSFETS
Substituting a logic level MOSFET for Q
3
(such as a BS170) results in lower system
voltage losses and significantly reduces output loading on the TC64X and TC64XB
devices. The low R
DSON
of the MOSFET (1Ω in the case of the BS170) enables it to be
used instead of the Darlington configuration in high-current fan applications. Refer to
the Applications section of the TC64X and TC64XB data sheets for details.
Population options for single transistor output drivers (using either MOSFET or bipolar
transistor) and the two transistor (Darlington) stage are shown in Figure 2-1.
FIGURE 2-1: Output Driver Configuration Options
2.2 SYSTEM CONNECTION FOR +5V OPERATION
Figure 2-2 shows typical wiring connections to the fan control module. The fan
operating voltage is +12V, while the fan control module operates from a supply voltage
of +5V. A NTC thermistor connects from the SENSOR input of the fan control module
to V
DD
. Fan control module resistors R
1
and R
2
(See Figure A-2), in conjunction with
characteristics of the NTC thermistor, determine the fan speed versus the temperature
profile of the system.
FIGURE 2-2: Typical Wiring Connections to Fan Module for +12V Fan and +5V Module Supply
C
7
Q
1
R
5
Q
3
C
B
E
Q
2
C
7
Q
1
Q
3
Reverse Orientation of
Q
3
and Jumper R
5
Q
2
C
7
Q
1
R
5
Q
3
Q
2
D
G
S
C
B
E
C
B
E
7
R
5
R
6
To Fan (–)
Q
1
Q
2
7
Jumper
R
5
R
6
Q
3
(BS170)
Q
1
, Q
2
= 2N2222A
7
R
5
R
6
To F a n ( – )
To F a n (– )
Q
3
(2N2222A)
R
5
V
DD
TC64X/TC64XB
Demo
Fan Control
Module
Power Supply
SENSOR
V
AS
/V
MIN
FAULT
System
Shutdown
NTC
Thermistor
Fan
FAN(–)
GND
+5V
+12V
GND
+
