Datasheet

TC646B/TC648B/TC649B
DS21755C-page 22  2002-2013 Microchip Technology Inc.
FIGURE 5-7: Output Drive Device Configurations.
TABLE 5-2: FAN DRIVE DEVICE SELECTION TABLE (NOTE 2)
5.6 Bias Supply Bypassing and Noise
Filtering
The bias supply (V
DD
) for the TC646B/TC648B/
TC649B devices should be bypassed with a 1.0 µF
ceramic capacitor. This capacitor will help supply the
peak currents that are required to drive the base/gate
of the external fan drive devices.
As the V
IN
pin controls the duty cycle in a linear fashion,
any noise on this pin can cause duty cycle jittering. For
this reason, the V
IN
pin should be bypassed with a
0.01 µF capacitor.
In order to keep fan noise off of the TC646B/TC648B/
TC649B device ground, individual ground returns for
the TC646B/TC648B/TC649B and the low side of the
fan current sense resistor should be used.
5.7 Design Example/Typical
Application
The system has been designed with the following
components and criteria:
System inlet air ambient temperature ranges from 0ºC
to 50ºC. At 20ºC, system cooling is no longer required,
so the fan is to be turned off. Prior to turn-off, the fan
should be run at 40% of its full fan speed. Full fan
speed should be reached when the ambient air is 40ºC.
The system has a surface mount, NTC-style thermistor
in a 1206 package. The thermistor is mounted on a
daughtercard that is directly in the inlet air stream. The
thermistor is a NTC, 100 k @ 25ºC, Thermometrics
®
part number NHQ104B425R5. The given Beta for the
thermistor is 4250. The system bias voltage to run the
fan controller is 5V, while the fan voltage is 12V.
Device Package
Max Vbe sat /
Vgs(V)
Min hfe
V
CE
/V
DS
(V)
Fan Current
(mA)
Suggested
Rbase ()
MMBT2222A SOT-23 1.2 50 40 150 800
MPS2222A TO-92 1.2 50 40 150 800
MPS6602 TO-92 1.2 50 40 500 301
SI2302 SOT-23 2.5 NA 20 500 Note 1
MGSF1N02E SOT-23 2.5 NA 20 500 Note 1
SI4410 SO-8 4.5 NA 30 1000 Note 1
SI2308 SOT-23 4.5 NA 60 500 Note 1
Note 1: A series gate resistor may be used in order to control the MOSFET turn-on and turn-off times.
2: These drive devices are suggestions only. Fan currents listed are for individual fans.
Q
1
GND
Fan Bias
R
SENSE
R
BASE
V
OUT
FAN
a) Single Bipolar Transistor
Q
1
GND
Fan Bias
R
SENSE
V
OUT
b) N-Channel MOSFET
FAN