TC647
DS21447D-page 14 2001-2012 Microchip Technology Inc.
FIGURE 5-7: Output Drive Transistor Circuit Topologies.
FIGURE 5-8: Powering the Fan from a
-12V Supply.
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
MIN
divider or shutdown circuit) is
powered by a supply different from that of the TC647.
Care should be taken to ensure that the TC647’s V
DD
supply powers up first. If possible, the networks
attached to V
IN
and V
MIN
should connect to the V
DD
supply 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 connect-
ing 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
MIN
inputs may cause
erroneous operation of the FAULT
output. As a result,
these inputs should be bypassed with a 0.01 µF capac-
itor mounted as close to the package as possible. This
is particularly true of V
IN
, which is usually driven from a
high impedance source (such as a thermistor). In addi-
tion, the V
DD
input should be bypassed with a 1 µF
capacitor. Ground should be kept as short as possible.
To keep fan noise off the TC647 ground pin, individual
ground returns for the TC647 and the low side of the
fan current sense resistor should be used.
Design Example
Step 1. Calculate R
1
and R
2
based on using an NTC
having a resistance of 10 k at T
MIN
(25°C)
and 4.65 k at T
MAX
(45°C) (see Figure 5-9).
R
1
= 20.5 k
R
2
= 3.83 k
Step 2. Set minimum fan speed V
MIN
= 1.8V.
Limit the divider current to 100 µA from which
R
5
= 33 k and R
6
= 18 k
Step 3. Design the output circuit.
Maximum fan motor current = 250 mA.
Q
1
beta is chosen at 50 from which
R
7
= 800
Q
1
Q
1
Q
2
GND
V
DD
R
SENSE
R
BASE
V
OUT
V
OUT
Fan
a) Single Bipolar Transistor
Q
1
GND
V
DD
R
SENSE
V
OUT
C) N-Channel MOSFET
GND
V
DD
R
SENSE
R
BASE
Fan
b) Darlington Transistor Pair
Fan
GND
+5V
Q
1
*
V
DD
V
OUT
TC647
Fan
R
2
*
2.2 kΩ
R
3
*
2.2 Ω
*Note: Value depends on the specific application and is shown for example only.
D
1
12.0V
Zener
-12V
R
4
*
10 kΩ
