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MMBT2222A View Datasheet(PDF) - Microchip Technology

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MMBT2222A Datasheet PDF : 28 Pages
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TC642
5.7 Power Supply Routing and
Bypassing
Noise present on the VIN and VMIN inputs may cause
erroneous operation of the FAULT output. As a result,
these inputs should be bypassed with a 0.01 µF
capacitor mounted as close to the package as is possi-
ble. This is particularly true of VIN, which is usually
driven from a high impedance source (such as a ther-
mistor). In addition, the VDD input should be bypassed
with a 1 µF capacitor. Grounds should be kept as short
as possible. To keep fan noise off the TC642 ground
pin, individual ground returns for the TC642 and the low
side of the fan current sense resistor should be used.
Design Example
Step 1. Calculate R1 and R2 based on using an NTC
having a resistance of 10 kat TMIN (25°C)
and 4.65 kat TMAX (45°C) (see Figure 5-9).
R1 = 20.5 k
R2 = 3.83 k
Step 2. Set minimum fan speed VMIN = 1.8V.
Limit the divider current to 100 µA from which
R5 = 33 kand R6 = 18 kΩ.
Step 3. Design the output circuit.
Maximum fan motor current = 250 mA.
Q1 beta is chosen at 50 from which
R7 = 800 .
Fan
Shutdown
R8
10 k
(Optional)
+5V
+5V
R1
20.5 k
R2
3.83 k
NTC CB +
10 k1 µF
@ 25˚C
1
VIN
CB
0.01 µF
8
VDD
4
GND
6
FAULT
+5V
R5
33 k
3 VMIN
CB
0.01 µF
Q2
R6
18 k
2
CF
C1
1µF
TC642
VOUT 7
5
SENSE
Fan/
Thermal Fault
CSENSE
0.1 µF
+12V
Fan
Q1
R7
800
RSENSE
2.2
FIGURE 5-9:
Design Example.
2002 Microchip Technology Inc.
DS21444C-page 15
 

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