TC646 Datasheet, PDF (14/28 Pages) Microchip Technology – PWM Fan Speed Controller with Auto-Shutdown and FanSense™ Technology

TC646
A base-current limiting resistor is required with bipolar
transistors. This is shown in Figure 5-6.
V_DD
Fan
V_OH = 80% V_DD
R_BASE
+ _VRBASEVB^–+_E(SAT)–
+
V_RSENSE
–
Q₁
R_SENSE
FIGURE 5-6:
R_BASE.
GND
Circuit For Determining
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
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
R_BASE = ^{VOH - VBE(SAT) - VRSENSE}
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. Zener diode D₁ offsets 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.
V_DD
V_DD
V_DD
Fan
Fan
Fan
V_OUT
R_BASE
R_BASE
V_OUT
Q₁
R_SENSE
Q₁
Q₂
V_OUT
R_SENSE
Q₁
R_SENSE
GND
a) Single Bipolar Transistor
GND
b) Darlington Transistor Pair
FIGURE 5-7:
Output Drive Transistor Circuit Topologies.
GND
c) N-Channel MOSFET
DS21446C-page 14
 2002 Microchip Technology Inc.