Part # Application Note Power Supply And Power Management TSM101 AN921 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics

Part Description: SM101 used in a thermostat

Part Details:

® APPLICATION NOTE TSM101 USED IN A THERMOSTAT by R. LIOU This technical note shows how to use the TSM101 Figure 1 : TSM101 Schematic Diagram integrated circuit to realize a simple Thermostatcontrolling a fan in cooling applications. An example of realization is given with the corre-sponding calculations. TSM101 PRESENTATION The TSM101 integrated circuit incorporates a highstability series band gap voltage reference (1.24V2% precision or 1% with the A version), two ORedoperational amplifiers and a current source(1.4mA) as shown on Figure 1. APPLICATION CONTEXT and PRINCIPLE OFOPERATION A temperature controlled fan is a common applica-tion in all electronic systems confined in a reducedand enclosed volume. As an example, the TSM101can be used in such applications for Mother Boardscooling, or in SMPS (Switch Mode Power Sup- Figure 2 : Basic Thermostat Function plies). This Thermostat is to be used in association with atemperature sensor (ex LM335). One Operationalamplifier of the TSM101 compares the voltagegiven by the temperature sensor to an internalprecision voltage reference, and as soon as thetemperature is higher than the preset limit, theoutput 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 thisapplication. Resistor bridge R1/R2 sets the tem-perature limit. C1 stabilizes the output voltage ofthe Temperature sensor. R3 ensures the hysteresisof the system. An improvement is shown on figure 3 where thecurrent source is used to supply the Temperaturesensor (with 1.4mA cathode current). This requiresthat pin 2 and pin 5 be connected to the systemground. This saves the polarization resistor of thetemperature sensor. AN921/0299 1/3 APPLICATION NOTE Figure 3 : The Internal Current Source Can CALCULATION OF THE ELEMENTS Supply the Temperature Sensor The following calculations apply to an Overcurrentand Overtemperature Fan Controller (figure 4). Temperature Control : The temperature upper limit is determined by theresistor bridge R1/R2.· Vref = Vsensor (T°) x R2 / (R1 + R2)If the sensor is an LM335, then the voltage functionof the temperature is a direct translation of thetemperature in Kelvin degrees following :· V (T°) = T° (°K) / 100.As an example, at 25°C, the output voltage of theLM335 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 = 30k as a good compromise precision/consumption. An other improvement can be achieved for Switch · R2 = 12k, R1 = 18k starts the fan at 50°C. Mode Power Supplies (SMPS) where it is useful tostart the cooling device as soon as the temperature · C1 = 0.1µF stabilizes the LM335 output. is too high OR when the overall current is above a · R = 10k supplies the LM335. preset limit (the cooling device can therefore antici-pate on the temperature elevation). This is shown Current Control : on figure 4 where the drop voltage across the sense The current limit is determined by the resistor resistor R8 is compared to a set limit given by the bridge R6/R7 and the sense resistor R8. resistor bridge R6/R7. In this configuration, the fan In many applications where it is necessary to re- motor is started either by an overtemperature, or duce temperature with air flow, a common current by an overcurrent. scale is in between 1 and 10 amps. Figure 4 : An Overcurrent and Overtemperature Fan Control 2/3 APPLICATION NOTE

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Application Note Power Supply And Power Management TSM101 AN921.pdf Datasheet