TC646BEPA Datasheet, PDF (22/36 Pages) Microchip Technology – PWM Fan Speed Controllers With Auto-Shutdown

TC646B/TC648B/TC649B
Fan Bias
Fan Bias
FAN
FAN
V_OUT
R_BASE
Q₁
R_SENSE
V_OUT
Q₁
R_SENSE
FIGURE 5-7:
GND
a) Single Bipolar Transistor
Output Drive Device Configurations.
GND
b) N-Channel MOSFET
TABLE 5-2: FAN DRIVE DEVICE SELECTION TABLE (NOTE 2)
Device
Package
Max Vbe sat /
Vgs(V)
Min hfe
V_CE/V_DS
(V)
Fan Current
(mA)
Suggested
Rbase (Ω)
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.
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 kΩ @ 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.
DS21755B-page 22
 2003 Microchip Technology Inc.