®
APPLICATION NOTE
TSM101 USED IN A THERMOSTAT
by R. LIOU
This technical note shows how to use the TSM101
integrated circuit to realize a simple Thermostat
controlling a fan in cooling applications.
An example of realization is given with the corre-
sponding calculations.
TSM101 PRESENTATION
The TSM101 integrated circuit incorporates a high
stability series band gap voltage reference (1.24V
2% precision or 1% with the A version), two ORed
operational amplifiers and a current source
(1.4mA) as shown on Figure 1.
APPLICATION CONTEXT and PRINCIPLE OF
OPERATION
A temperature controlled fan is a common applica-
tion in all electronic systems confined in a reduced
and enclosed volume. As an example, the TSM101
can be used in such applicationsfor Mother Boards
cooling, or in SMPS (Switch Mode Power Sup-
plies).
This Thermostat is to be used in association with a
temperature sensor (ex LM335). One Operational
amplifier of the TSM101 compares the voltage
given by the temperature sensor to an internal
precision voltage reference, and as soon as the
temperature is higher than the preset limit, the
output of the operational amplifier is set active
(active low) and can switch a fan motor ON.
Figure 2 shows how TSM101 is to be used in this
application. Resistor bridge R1/R2 sets the tem-
perature limit. C1 stabilizes the output voltage of
the Temperature sensor. R3 ensures the hysteresis
of the system.
An improvement is shown on figure 3 where the
current source is used to supply the Temperature
sensor (with 1.4mAcathode current). This requires
that pin 2 and pin 5 be connected to the system
ground. This saves the polarization resistor of the
temperature sensor.
AN921/0299
Figure 1 :
TSM101 Schematic Diagram
Figure 2 :
Basic Thermostat Function
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APPLICATION NOTE
Figure 3 :
The Internal Current Source Can
Supply the Temperature Sensor
CALCULATION OF THE ELEMENTS
The following calculations apply to an Overcurrent
and Overtemperature Fan Controller (figure 4).
Temperature Control :
The temperature upper limit is determined by the
resistor bridge R1/R2.
•
Vref = Vsensor (T°) x R2 / (R1 + R2)
If the sensor is an LM335, then the voltage function
of the temperature is a direct translation of the
temperature in Kelvin degrees following :
•
V (T°) = T° (°K) / 100.
As an example, at 25°C, the output voltage of the
LM335 is (273 + 25) / 100=2.98V.
Let us assume that an acceptable upper tempera-
ture limit is 50°C, therefore :
•
1.24 = 3.23 x R2 / (R1+ R2) with R1+ R2 = 30kΩ
as a good compromise precision/consumption.
An other improvement can be achieved for Switch
Mode Power Supplies (SMPS) where it is useful to
start the cooling device as soon as the temperature
is too high OR when the overall current is above a
preset limit (the cooling device can therefore antici-
pate on the temperature elevation). This is shown
on figure 4 where the drop voltage across the sense
resistor R8 is compared to a set limit given by the
resistor bridge R6/R7. In this configuration, the fan
motor is started either by an overtemperature, or
by an overcurrent.
Figure 4 :
•
R2 = 12kΩ, R1 = 18kΩ starts the fan at 50°C.
•
C1 = 0.1µF stabilizes the LM335 output.
•
R = 10kΩ supplies the LM335.
Current Control :
The current limit is determined by the resistor
bridge R6/R7 and the sense resistor R8.
In many applications where it is necessary to re-
duce temperature with air flow, a common current
scale is in between 1 and 10 amps.
An Overcurrent and Overtemperature Fan Control
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APPLICATION NOTE
Let us assume that TSM101 is used as a thermo-
stat for an application which has a maximum con-
sumption of 10A and requires air flow starting at a
current consumption of 2A.
The voltage drop through the sense resistor R8 is
given by :
•
Vdrop = Vref x R7 / (R6 + R7)
At 10A, a tolerable voltage drop can be chosen as
50mV, therefore, the voltage drop corresponding to
2A is 10mV.
•
0.01 = 1.24 x R7 / (R6 + R7) where R6 + R7 ~
1.2kΩ to ensure proper charge for the voltage
reference.
•
R7 = 10Ω, R6 = 1.2kΩ
The sense resistor R8 determines the upper cur-
rent limit following :
•
Vdrop = R8 x Imax
•
R8 = 5mΩ
Motor Control :
The power transistor Q1 is controlled via its base
resistor R4. To limit the base current to 10mA, R4
should be chosen in the range of 1kΩ. The pull up
resistor R5 can be chosen in the range of 10kΩ.
•
R4 = 1kΩ, R5 = 10kΩ
The hysteresis on the temperature is determined
by R3 : when Q1 is ON, the negative input voltage
of the operational amplifier is pulled up ; when it is
OFF, the negative input is pulled down. To make a
unidirectional hysteresis, a diode is inserted in
series with R3 in order to achieve hysteresis when
the fan is ON. R3 must be greater than R2 // R1 not
to offset the measurement of the temperature sen-
sor.
•
R2 // R1 = R1 x R2 / (R1 + R2) = 7.2kΩ = Req
Let usassume that the precision of the temperature
control is in the range of +/- 1°C, and that we expect
a 5°C hysteresis.
On the input (pin 7) of TSM101, 1°C corresponds
to 10mV x R2 / (R1 + R2) = 4mV, therefore,5°C will
correspond to Vhyst = 20mV.
The resistor R3 should have an influence of 20mV
on pin 7, following the equation :
•
Vcc x Req / (R3 + Req) = Vhyst
•
R3 = 4.3MΩ
The capacitors C2 and C3 stabilize respectively the
command of the power transistor and filter the
noise due to the motor.
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the
consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from
its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications
mentioned in this publication are subject to change without notice. This publ ication supersedes and replaces all information
previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems
without express written approval of STMicroelectronics.
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