Datasheet

TC646
DS21446D-page 14  2002-2012 Microchip Technology Inc.
A base-current limiting resistor is required with bipolar
transistors. This is shown in Figure 5-6.
FIGURE 5-6: Circuit For Determining
R
BASE
.
The correct value for this resistor can be determined as
follows:
V
OH
= V
R
SENSE
+ V
BE
(SAT)
+ V
R
BASE
V
R
SENSE
= I
FAN
x R
SENSE
V
R
BASE
= R
BASE
x I
BASE
I
BASE
= I
FAN
/ h
FE
V
OH
is specified as 80% of V
DD
in Section 1.0,
“Electrical Characteristics”; V
BE
(SAT)
is given in the
chosen transistor data sheet. It is now possible to solve
for R
BASE
.
EQUATION
Some applications benefit from the fan being powered
from a negative supply to keep motor noise out of the
positive supply rails. This can be accomplished as
shown in Figure 5-8. Zener diode D
1
offsets the -12V
power supply voltage, holding transistor Q
1
off when
V
OUT
is low. When V
OUT
is high, the voltage at the
anode of D
1
increases by V
OUT
, causing Q
1
to turn on.
Operation is otherwise the same as in the case of fan
operation from +12V.
FIGURE 5-7: Output Drive Transistor Circuit Topologies.
Q
1
GND
V
DD
R
SENSE
R
BASE
V
OH
= 80% V
DD
+
V
R
BASE
–
V
BE
(SAT)
–
+
V
R
SENSE
–
Fan
+
R
BASE
=
V
OH
- V
BE
(SAT)
- V
R
SENSE
I
BASE
Q
1
Q
1
Q
2
GND
V
DD
R
SENSE
R
BASE
V
OUT
V
OUT
Fan
a) Single Bipolar Transistor
Q
1
GND
V
DD
R
SENSE
V
OUT
c) N-Channel MOSFET
GND
V
DD
R
SENSE
R
BASE
Fan
b) Darlington Transistor Pair
Fan