TC649 Microchip, TC649 Datasheet - Page 15

• Manufacturer Part Number: TC649
• Description: PWM Fan Speed Controller
• Manufacturer: Microchip

5.6
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 TC649.
Care should be taken to ensure that the TC649’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.
5.7
Noise present on the V
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
FIGURE 5-9:
2002 Microchip Technology Inc.
Shutdown
RESET
Latch-up Considerations
Power Supply Routing and
Bypassing
(Optional)
IN
and V
AS
Open-
Drain
Device
AS
divider or shutdown circuit) are
Design Example.
should connect to the V
IN
IN
and V
, which is usually drive from
+5V
R 1
20.5 k
AS
R 2
3.83 k
+5V
inputs may cause
R 6
18 k
R 5
33 k
C
0.01 µF
C
0.01 µF
B
@ 25˚C
10 k
B
C 1
1 µF
NTC
DD
3
1
2
sup-
V
V
C
DD
AS
IN
F
C
1 µF
B
+5V
V
DD
TC649
8
a high impedance source (such as a thermistor). Addi-
tionally, the V
capacitor with grounds being kept as short as possible.
To keep fan noise off the TC649 ground pin, individual
ground returns for the TC649 and the low side of the
fan current sense resistor should be used.
Design Example
Step 1. Calculate R
Step 2. Set auto-shutdown Level.
Step 3. Design the output circuit.
GND
4
having a resistance of 10 k
and 4.65 k at T
R
Limit the divider current to 100 µA
R
R
V
Q
R
SENSE
R
Maximum fan motor current = 250 mA.
FAULT
V
AS
2
5
6
1
7
1
OUT
= 3.83 k
= 33 k
= 18 k
= 20.5 k
beta is chosen at 50 from which
= 800
= 1.8V.
DD
input should be bypassed with a 1 µF
5
6
7
1
Fan Fault
and R
C
0.1 µF
SENSE
MAX
2
based on using an NTC
(45°C) (see Figure 5-9).
DS21449C-page 15
TC649
R 7
800
at T
+12V
Fan
MIN
Q 1
R
2.2
SENSE
(25°C)