TC649 Datasheet, PDF (14/28 Pages) Microchip Technology – PWM Fan Speed Controller with Auto-Shutdown and FanSense™ Technology

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_OH
= V_RSENSE + V_BE(SAT) + V_RBASE
V_RSENSE = I_FAN x R_SENSE
V_RBASE
= 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 cho-
sen transistor data sheet. It is now possible to solve for
R_BASE.
EQUATION
V_OH - V_BE(SAT) - V_RSENSE
R_BASE =
I_BASE
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 D₁ offsetting the
-12V power supply voltage, holding transistor Q₁ off
when V_OUT is low. When V_OUT is high, the voltage at
the anode of D₁ increases by V_OUT, causing Q₁ to turn
on. Operation is otherwise the same as in the case of
fan operation from +12V.
+5V
V_DD
Fan
V_OH = 80% V_DD
R_BASE
+ _VRBASEVB^–+_E(SAT)–
+
V_RSENSE
–
Q1
R_SENSE
FIGURE 5-7:
R_BASE.
GND
Circuit For Determining
V_DD
V_OUT
TC649
R2*
2.2 kΩ
D1
12.0V
Zener
GND
R4*
10 kΩ
FAN
Q1*
R3*
2.2 Ω
Note: * Value depends on the specific application and is shown for example only.
See Section 5.0, "Typical Applications", for more details.
FIGURE 5-8:
Powering the Fan from a -12V Supply.
-12V
DS21449C-page 14
 2002 Microchip Technology Inc.