TC646 Microchip, TC646 Datasheet - Page 14

• Manufacturer Part Number: TC646
• Description: PWM Fan Speed Controller
• Manufacturer: Microchip

TC646
A base-current limiting resistor is required with bipolar
transistors. This is shown in Figure 5-6.
FIGURE 5-6:
R
FIGURE 5-7:
DS21446C-page 14
V
BASE
OUT
V
OH
.
= 80% V
R
a) Single Bipolar Transistor
BASE
DD
+ V
Circuit For Determining
Output Drive Transistor Circuit Topologies.
R
GND
V
Fan
R BASE
BASE
DD
Q
V
R
1
V
SENSE
BE (SAT)
–
R SENSE
+
+
V
–
GND
V
–
Fan
OUT
DD
R
Q
SENSE
1
R
BASE
b) Darlington Transistor Pair
The correct value for this resistor can be determined as
follows:
V
“Electrical Characteristics”; V
chosen transistor data sheet. It is now possible to solve
for R
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
power supply voltage, holding transistor Q
V
anode of D
Operation is otherwise the same as in the case of fan
operation from +12V.
Q
OH
OUT
1
GND
V
Fan
DD
BASE
is specified as 80% of V
is low. When V
V
V
V
I
Q
BASE
R
OH
R SENSE
R BASE
2
SENSE
R
.
BASE
1
increases by V
=
= V
= I
= R
= I
V
V
OH
FAN
FAN
OUT
R SENSE
BASE
OUT
- V
/ h
x R
2002 Microchip Technology Inc.
BE (SAT)
x I
FE
is high, the voltage at the
OUT
SENSE
I
BASE
+ V
BASE
BE (SAT)
, causing Q
c) N-Channel MOSFET
BE (SAT)
- V
DD
R SENSE
1
offsets the -12V
is given in the
in Section 1.0,
V
GND
+ V
Fan
DD
1
1
R BASE
to turn on.
Q
off when
R
1
SENSE