TC649 Microchip, TC649 Datasheet - Page 14

• Manufacturer Part Number: TC649
• Description: PWM Fan Speed Controller
• Manufacturer: Microchip

TC649
A base-current limiting resistor is required with bipolar
transistors (Figure 5-7). The correct value for this
resistor can be determined as follows:
V
“Electrical Characteristics”; V
sen 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, with zener diode D
-12V power supply voltage, holding transistor Q
when V
the anode of D
on. Operation is otherwise the same as in the case of
fan operation from +12V.
FIGURE 5-8:
DS21449C-page 14
OH
BASE
is specified as 80% of V
.
V
V
V
I
BASE
OUT
Note: * Value depends on the specific application and is shown for example only.
OH
R
R
R
SENSE
BASE
BASE
is low. When V
1
See Section 5.0, "Typical Applications", for more details.
=
increases by V
= V
= I
= R
= I
V
OH
FAN
FAN
R
BASE
Powering the Fan from a -12V Supply.
SENSE
- V
/ h
x R
BE(
x I
FE
OUT
I
SENSE
+ V
BE(
BASE
SAT
BASE
OUT
)
BE(
SAT
is high, the voltage at
TC649
- V
GND
)
SAT
DD
, causing Q
+5V
V
R
is given in the cho-
SENSE
DD
)
+ V
in Section 1.0,
1
offsetting the
R
BASE
1
V
OUT
to turn
1
off
12.0V
Zener
D 1
FIGURE 5-7:
R
BASE
V
R 2 *
2.2 k
OH
.
= 80% V
DD
R 4 *
10 k
+ V
Circuit For Determining
R
R BASE
BASE
2002 Microchip Technology Inc.
V
V
–
BE (SAT)
FAN
-12V
R SENSE
+
Q 1 *
R 3 *
2.2
+
–
GND
V
–
Fan
DD
R
Q 1
SENSE