Datasheet

TC646
DS21446D-page 16  2002-2012 Microchip Technology Inc.
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 auto-shutdown level V
AS
= 1.8V.
Limit the divider current to 100 µA from which
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 .
FIGURE 5-9: Design Example.
5.8 TC646 as a Microcontroller
Peripheral
In a system containing a microcontroller or other host
intelligence, the TC646 can be effectively managed as
a CPU peripheral. Routine fan control functions can be
performed by the TC646 without controller intervention.
The microcontroller receives temperature data from
one or more points throughout the system. It calculates
a fan operating speed based on an algorithm specifi-
cally designed for the application at hand. The proces-
sor controls fan speed using complementary port bits
I/O1 through I/O3. Resistors R
1
through R
6
(5% toler-
ance) form a crude 3-bit DAC that translates the 3-bit
code from the processor's outputs into a 1.6V DC con-
trol signal. A monolithic DAC or digital pot may be used
instead of the circuit shown in Figure 5-10.
With V
AS
set at 1.8V, the TC646 enters auto-shutdown
when the controller's output code is 000[B]. Output
codes 001[B] to 111[B] operate the fan from roughly
40% to 100% of full speed. An open-drain output from
the processor (I/O0) can be used to reset the TC646
following detection of a fault condition. The FAULT
out-
put can be connected to the controller's interrupt input,
or to another I/O pin, for polled operation.
FAULT
SENSE
NTC
10 kΩ
@ 25˚C
R
1
20.5 kΩ
R
2
3.83 kΩ
R
5
33 kΩ
R
7
800Ω
R
6
GND
Reset
Shutdown
(Optional)
Q
1
+12V
+5V
+5V
+5V
V
DD
V
IN
V
AS
V
OUT
R
SENSE
2.2Ω
C
SENSE
0.1 μF
C
1
1 μF
C
F
TC646
Fan
Fan/Thermal
Fault
C
B
0.01 μF
C
B
0.01 μF
C
B
1 μF
1
8
4
6
7
5
2
3
18 kΩ
Open-Drain
Device