Datasheet

 2002-2012 Microchip Technology Inc. DS21446D-page 15
TC646
FIGURE 5-8: Power the Fan from a -12V Supply.
TABLE 5-2: TRANSISTORS AND MOSFETS FOR Q
1
(V
DD
= 5V)
5.6 Latch-up Considerations
As with any CMOS IC, the potential exists for latch-up
if signals are applied to the device which are outside
the power supply range. This is of particular concern
during power-up if the external circuitry (such as the
sensor network, V
AS
divider or shutdown circuit) is
powered by a supply different from that of the TC646.
Care should be taken to ensure that the TC646’s V
DD
supply powers up first. If possible, the networks
attached to V
IN
and V
AS
should connect to the V
DD
sup-
ply at the same physical location as the IC itself. Even
if the IC and any external networks are powered by the
same supply, physical separation of the connecting
points can result in enough parasitic capacitance and/
or inductance in the power supply connections to delay
one power supply “routing” versus another.
5.7 Power Supply Routing and
Bypassing
Noise present on the V
IN
and V
AS
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 possible.
This is especially true of V
IN
, which is usually driven
from a high impedance source (such as a thermistor).
In addition, the V
DD
input should be bypassed with a
1 µF capacitor. Grounds should be kept as short as
possible. To keep fan noise off the TC646 ground pin,
individual ground returns for the TC646 and the low
side of the fan current sense resistor should be used.
Device Package
Max. V
BE(sat)
/V
GS
(V)
Min. H
FE
V
CEO
/V
DS
(V)
Fan Current
(mA)
Suggested
R
BASE
()
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.
GND
+5V
-12V
Q
1
*
V
DD
V
OUT
TC646
Fan
R
2
*
2.2 kΩ
R
3
*
2.2Ω
R
4
*
10 kΩ
D
1
12.0V
Zener
NOTE: *Value depends on the specific application and is shown for example only.