TC646
A base-current limiting resistor is required with bipolar
transistors. This is shown in Figure 5-6.
V
DD
The correct value for this resistor can be determined as
follows:
V
OH
V
RSENSE
V
RBASE
I
BASE
= V
RSENSE
+ V
BE(SAT)
+ V
RBASE
= I
FAN
x R
SENSE
= R
BASE
x I
BASE
= I
FAN
/ h
FE
Fan
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
.
R
BASE
V
OH
= 80% V
DD
+V
RBASE
+
V
BE(SAT)
–
EQUATION
–
Q
1
R
BASE
=
V
OH
- V
BE(SAT)
- V
RSENSE
I
BASE
+
V
RSENSE
–
R
SENSE
GND
FIGURE 5-6:
R
BASE
.
Circuit For Determining
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.
V
DD
V
DD
V
DD
Fan
Fan
Fan
R
BASE
V
OUT
Q
1
R
BASE
V
OUT
Q
1
Q
2
V
OUT
Q
1
R
SENSE
R
SENSE
R
SENSE
GND
GND
GND
a) Single Bipolar Transistor
b) Darlington Transistor Pair
c) N-Channel MOSFET
FIGURE 5-7:
Output Drive Transistor Circuit Topologies.
DS21446C-page 14
2002 Microchip Technology Inc.
MICROCHIP [ MICROCHIP TECHNOLOGY ]
