TC648 Microchip, TC648 Datasheet - Page 14

• Manufacturer Part Number: TC648
• Description: Fan Speed Controller
• Manufacturer: Microchip

TC648
FIGURE 5-7:
5.5
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
powered by a supply different from that of the TC648.
Care should be taken to ensure that the TC648’s V
supply powers up first. If possible, the networks
attached to V
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
Noise present on the V
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
high impedance source (such as a thermistor). Addi-
tionally, the V
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.
DS21448C-page 14
Latch-up Considerations
Power Supply Routing and
Bypassing
IN
DD
and V
input should be bypassed with a 1 µF
AS
AS
IN
divider or shutdown circuit) are
Powering the Fan from a -12V Supply.
NOTE: *Value depends on the specific application and is shown for example only.
, which is usually driven from a
should connect to the V
IN
and V
AS
TC648
GND
V
+5V
inputs may cause
DD
V
OUT
DD
sup-
DD
12.0V
Zener
D
1
R
2.2 kΩ
2
*
Auto-Shutdown Mode Design Example
Step 1. Calculate R
Step 2. Set auto-shutdown level.
5.7
Given:
Step 1: Calculate R
Step 2: Verify V
Step 3. Design the output circuit
10 kΩ
R
4
*
having a resistance of 10 k
and 4.65 k at T
R
R
7.87 k )
Calculate R2:
R
4.02 k )
V
R
V
Limit the divider current to 100 µA
R
R
Maximum fan motor current = 250 mA.
Q
R
Minimum Speed Mode Design
Example
Minimum speed = 40%(1.8V)
T
Thermistor = 100 k
RT
2
AS
MIN
1
2
MAX
1
5
6
7
1
= 3.83 k
MIN
= 20.5 k
= 33 k
= 18 k
= 800 .
beta is chosen at 50 from which
= 4.05 k
= 7.9 k
= 1.8V
= 30°C, T
= 2.64V
= 79.4 k , RT
MAX
Fan
-12V
Q
1
:
1
1
:
*
and R
(Use closest standard value:
MAX
(Use closest standard value:
MAX
2002 Microchip Technology Inc.
2
= 95°C
MAX
based on using an NTC
(45°C) (see Figure 5-8).
at 25°C
= 6.5 k
at T
MIN
(25°C)