Datasheet

 2001-2012 Microchip Technology Inc. DS21448D-page 11
TC648
thermistor resistance at T
MIN
. R
2
is then calculated to
deliver the desired speed at T
MIN
. The values for R
1
, R
2
and RT
1
are then checked at T
MAX
for 2.65V nominal.
It may be necessary to adjust the values of R
1
and R
2
after the initial calculation to obtain the desired results.
The design equations are:
EQUATION
EQUATION
EQUATION
Because the thermistor characteristics are fixed, it may
not be possible, in certain applications, to obtain the
desired values of V
MIN
and V
MAX
using the above
equations. In this case, the circuit in Figure 5-4 can be
used. Diode D
1
clamps V
IN
to the voltage required to
sustain minimum speed. The calculations of R
1
and
R
2
for the temperature sensor are identical to the
equation on the previous page.
FIGURE 5-4: Minimum Fan Speed Circuit.
5.3 Auto-Shutdown Temperature
Design
A voltage divider on V
AS
sets the temperature at which
the part is automatically shut down if the sensed
temperature at V
IN
drops below the set temperature at
V
AS
(i.e. V
IN
< V
AS
).
As with the V
IN
input, 1.25V to 2.65V corresponds to
the temperature range of interest from T
1
to T
2
,
respectively. Assuming that the temperature sensor
network designed previously is linearly related to
temperature, the shutdown temperature T
AS
is related
to T
2
and T
1
by:
EQUATION
For example, if 1.25V and 2.65V at V
IN
corresponds to
a temperature range of T
1
= 0°C to T
2
= 125°C, and the
auto-shutdown temperature desired is 25°C, then the
V
AS
voltage is:
EQUATION
The V
AS
voltage may be set using a simple resistor
divider, as shown in Figure 5-5.
FIGURE 5-5: V
AS
Circuit.
R
1
= (0.1)(RT
1MIN
)
Where: RT
1
= Thermistor resistance at T
MIN
R
2
=
(RT
1MIN
)(R
1
)(V
MIN
)
(RT
1MIN
+ R
1
)(V
DD
- V
MIN
)
Where V
MIN
= the value of V
IN
required for
minimum fan speed. V
DD
= Power Supply Voltage
V
MAX
=
(RT
1MIN
)(R
1
)(V
MIN
)
R
2
(R
1
+ RT
1MAX
)(V
DD
)
Where RT
1MAX
= thermistor resistance at T
MAX
,
V
MAX
= the value of V
IN
required for maximum
fan speed.
R
1
R
2
R
3
R
4
D
1
RT
1
V
DD
V
IN
2.65 - 1.25V
T
2
- T
1
=
V
AS
- 1.25
T
AS
- T
1
V
AS
=
1.4V
T
2
- T
1
(T
AS
- T
1
) + 1.25
(
)
1.4V
(125 - 0)
V
AS
= (25 - 0) + 1.25 = 1.53V
R
2
R
1
GND
V
DD
V
AS
I
DIV
I
IN