TC648
DS21448D-page 14 2001-2012 Microchip Technology Inc.
FIGURE 5-7: Powering the Fan from a -12V Supply.
5.5 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) are
powered by a supply different from that of the TC648.
Care should be taken to ensure that the TC648’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.
5.6 Power Supply Routing and
Bypassing
Noise present on the V
IN
and V
AS
inputs may cause
erroneous operation of the OTF
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 especially true of V
IN
, which is usually driven from a
high impedance source (such as a thermistor). Addi-
tionally, the V
DD
input should be bypassed with a 1 µF
capacitor and grounds should be kept as short as pos-
sible. To keep fan noise off the TC648 ground pin, indi-
vidual ground returns for the TC648 and the low side of
the fan drive device should be used.
Auto-Shutdown Mode 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 5-8).
R
1
= 20.5 k
R
2
= 3.83 k
Step 2. Set auto-shutdown level.
V
AS
= 1.8V
Limit the divider current to 100 µA
R
5
= 33 k
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 .
5.7 Minimum Speed Mode Design
Example
Given:
Minimum speed = 40%(1.8V)
T
MIN
= 30°C, T
MAX
= 95°C
Thermistor = 100 k at 25°C
RT
MIN
= 79.4 k, RT
MAX
= 6.5 k
Step 1: Calculate R
1
:
R
1
= 7.9 k (Use closest standard value:
7.87 k)
Calculate R2:
R
2
= 4.05 k (Use closest standard value:
4.02 k)
Step 2: Verify V
MAX
:
V
MAX
= 2.64V
GND
+5V
-12V
Q
1
*
V
DD
V
OUT
TC648
Fan
R
2
*
2.2 kΩ
R
4
*
D
1
NOTE: *Value depends on the specific application and is shown for example only.
12.0V
Zener
10 kΩ
