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TC648VOA713

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TC648
DS21448D-page 10 2001-2012 Microchip Technology Inc.
5.1 Temperature Sensor Design
The temperature signal connected to V
IN
must output a
voltage in the range of 1.25V to 2.65V (typical) for 0%
to 100% of the temperature range of interest. The
circuit in Figure 5-2 illustrates a convenient way to pro-
vide this signal using a temperature dependent voltage
divider circuit.
FIGURE 5-2: Temperature Sensing
Circuit.
RT
1
is a conventional NTC thermistor and R
1
and R
2
are standard resistors. The supply voltage (V
DD
) is
divided between R
2
and the parallel combination of
RT
1
and R
1
. For convenience, the parallel combination
of RT
1
and R
1
will be referred to as R
TEMP
. The resis-
tance of the thermistor at various temperatures is
obtained from the manufacturer’s specifications.
Thermistors are often referred to in terms of their resis-
tance at 25°C.
Generally, the thermistor shown in Figure 5-2 is a non-
linear device with a negative temperature coefficient
(also called an NTC thermistor). In Figure 5-2, R
1
is
used to linearize the thermistor temperature response
and R
2
is used to produce a positive temperature
coefficient at the V
IN
node. As an added benefit, this
configuration produces an output voltage delta of 1.4V,
which is well within the range of the V
C(SPAN)
specification of the TC648. A 100 kNTC thermistor is
selected for this application in order to keep I
DIV
to a
minimum.
For the voltage range at V
IN
to be equal to 1.25V to
2.65V, the temperature range of this configuration is
0°C to 50°C. If a different temperature range is required
from this circuit, R
1
should be chosen to equal the
resistance value of the thermistor at the center of this
new temperature range. It is suggested that a maxi-
mum temperature range of 50°C be used with this cir-
cuit due to thermistor linearity limitations. With this
change, R
2
is adjusted according to the following
equations:
EQUATION
These two equations facilitate solving for the two
unknown variables, R
1
and R
2
. More information about
thermistors may be obtained from AN679, “Tempera-
ture Sensing Technologies”, and AN685, “Thermistors
in Single Supply Temperature Sensing Circuits”, which
can be downloaded from Microchip's web site at
www.microchip.com.
5.2 Minimum Speed Mode
The TC648 is configured for minimum speed mode by
grounding V
AS
and designing the temperature sensor
network such that V
IN
operates the fan at relatively con-
stant, minimum speed when the thermistor is at
minimum temperature. Figure 5-3 shows operation in
minimum speed mode. The 0% and 100% fan speeds
correspond to V
IN
values of 1.25V and 2.65V, typical.
Minimum system temperature (T
MIN
) is defined as the
lowest measured temperature at which proportional fan
speed control is required by the system. The fan
operates at minimum speed for all temperatures below
T
MIN
and at speeds proportional to the measured
temperature between T
MIN
and T
MAX
.
FIGURE 5-3: Minimum Fan Speed Mode
Operation.
Temperature sensor design consists of a two-point
calculation: one at T
MIN
and one at T
MAX
. At T
MIN
, the
ohmic value of the thermistor must be much higher
than that of R
1
so that minimum speed is determined
primarily by the values of R
1
and R
2
. At T
MAX
, the
ohmic value of the thermistor must result in a V
IN
of
2.65V nominal. The design procedure consists of ini-
tially choosing R
1
to be 10 times smaller than the
R
2
= 23.2 kΩ
R
1
= 100 kΩ
I
DIV
V
IN
V
DD
NTC Thermistor
100 kΩ @25˚C
RT
1
V
DD
x R
2
R
TEMP
(T
1
) + R
2
= V(T
1
)
R
TEMP
(T
2
) + R
2
= V(T
2
)
V
DD
x R
2
Where T
1
and T
2
are the chosen temperatures and
R
TEMP
is the parallel combination of the thermistor
and R
1
.
Fan Speed
100%
Minimum
Speed
0%
T
MIN
T
MAX
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
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
(
)
R
2
R
1
GND
V
DD
V
AS
I
DIV
I
IN
TC648
DS21448D-page 12 2001-2012 Microchip Technology Inc.
Per Section 1.0, “Electrical Characteristics”, the leak-
age current at the V
AS
pin is no more than 1 µA. It is
conservative to design for a divider current, I
DIV
, of
100 µA. If V
DD
= 5.0V then…
EQUATION
We can further specify R
1
and R
2
by the condition that
the divider voltage is equal to our desired V
AS
. This
yields the following:
EQUATION
Solving for the relationship between R
1
and R
2
results
in the following equation:
EQUATION
For this example, R
1
= (2.27) R
2
. Substituting this rela-
tionship back into the original equation yields the
resistor values:
R
2
= 15.3 k, and R
1
= 34.7 k
In this case, the standard values of 34.8 k and
15.4 k are very close to the calculated values and
would be more than adequate.
5.4 Output Drive Transistor Selection
The TC648 is designed to drive an external transistor
or MOSFET for modulating power to the fan. This is
shown as Q
1
in Figures 5-1, 5-6, 5-7,and 5-8. The
V
OUT
pin has a minimum source current of 5 mA and a
minimum sink current of 1 mA. Bipolar transistors or
MOSFETs may be used as the power switching ele-
ment, as is shown in Figure 5-6. When high current
gain is needed to drive larger fans, two transistors may
be used in a Darlington configuration. These circuit
topologies are shown in Figure 5-6: (a) shows a single
NPN transistor used as the switching element; (b) illus-
trates the Darlington pair; and (c) shows an N-channel
MOSFET.
One major advantage of the TC648’s PWM control
scheme versus linear speed control is that the power
dissipation in the pass element is kept very low.
Generally, low cost devices in very small packages,
such as TO-92 or SOT, can be used effectively. For
fans with nominal operating currents of no more than
200 mA, a single transistor usually suffices. Above
200 mA, the Darlington or MOSFET solution is
recommended. For the power dissipation to be kept
low, it is imperative that the pass transistor be fully sat-
urated when "on".
Table 5-1 gives examples of some commonly available
transistors and MOSFETs. This table should be used
as a guide only since there are many transistors and
MOSFETs which will work just as well as those listed.
The critical issues when choosing a device to use as
Q1 are: (1) the breakdown voltage (V
(BR)CEO
or V
DS
(MOSFET)) must be large enough to withstand the
highest voltage applied to the fan (Note: This will occur
when the fan is off); (2) 5 mA of base drive current must
be enough to saturate the transistor when conducting
the full fan current (transistor must have sufficient
gain); (3) the V
OUT
voltage must be high enough to suf-
ficiently drive the gate of the MOSFET to minimize the
R
DS(on)
of the device; (4) rated fan current draw must
be within the transistor's/MOSFET's current handling
capability; and (5) power dissipation must be kept
within the limits of the chosen device.
A base-current limiting resistor is required with bipolar
transistors. The correct value for this resistor can be
determined as follows:
V
OH
=V
BE
(SAT)
+ V
R
BASE
V
R
BASE
=R
BASE
x I
BASE
I
BASE
=I
FAN
/ h
FE
V
OH
is specified as 80% of V
DD
in Section 1.0,
“Electrical Characteristics”; V
BE
(SAT)
is given in the
chosen transistor data sheet. It is now possible to solve
for R
BASE
.
EQUATION
Some applications benefit from the fan being powered
from a negative supply to keep motor noise out of the
positive supply rails. This can be accomplished by the
method shown in Figure 5-7. Zener diode D
1
offsets
the -12V power supply voltage, holding transistor Q
1
off
when V
OUT
is low. When V
OUT
is high, the voltage at
the anode of D
1
increases by V
OH
, causing Q
1
to turn
on. Operation is otherwise the same as in the case of
fan operation from +12V.
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TC648VOA713

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