Datasheet

 2002-2013 Microchip Technology Inc. DS21756C-page 21
TC642B/TC647B
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 TC642B/TC647B devices
should be bypassed with a 1.0 F 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 TC642B/TC647B
device ground, individual ground returns for the
TC642B/TC647B 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 and below, it is desired to have the
system cooling stay at a constant level. At 20ºC, 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, which is directly in the inlet air stream.
The thermistor is a NTC, 100 k @ 25ºC, Thermomet-
rics
®
part number NHQ104B425R5. The given Beta for
the thermistor is 4250. The system bias voltage to run
the fan controller is 5V and 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