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TMP709 Datenblatt(PDF) 6 Page - Texas Instruments |
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TMP709 Datenblatt(HTML) 6 Page - Texas Instruments |
6 / 14 page Hysteresis Control Positive Tempco Reference HYST SET Negative Tempco Reference TMP709 R SET R PULLUP +2.7 V to +5.5 V OT VCC GND TMP709 SBOS583A – DECEMBER 2011 – REVISED FEBRUARY 2012 www.ti.com THEORY OF OPERATION DEVICE DESCRIPTION The TMP709 is a fully-integrated, resistor-programmable temperature switch that incorporates two temperature-dependent voltage references and one comparator. One voltage reference exhibits a positive temperature coefficient (tempco), and the other voltage reference exhibits a negative tempco. The temperature at which both voltage references are equal determines the temperature trip point. The TMP709 temperature threshold is programmable from 0 °C to +125°C and is set by an external 1% resistor from the SET pin to the GND pin. The TMP709 has an open-drain, active-low output structure that can easily interface with a microprocessor. HYSTERESIS INPUT The HYST pin is a digital input that allows the input hysteresis to be set at either 10 °C (when HYST = VCC) or 2 °C (when HYST = GND). The hysteresis function keeps the OT pin from oscillating when the temperature is near the threshold. Thus, the HYST pin should always be connected to either VCC or GND. Other input voltages on this pin may cause abnormal supply currents and/or function. CIRCUIT DETAILS Figure 5 shows the comparator, the NFET open-drain device connected to the OT pin, the positive tempco reference using the external RSET resistor, the negative tempco reference, and the hysteresis control. The voltage of the positive tempco reference is controlled by external resistor RSET. Figure 5. Circuit Details The TMP709 reaches the temperature trip point when the voltage from the positive tempco reference is greater than the voltage from the negative tempco reference. This situation causes the output of the comparator to switch from logic 0 to logic 1. The comparator output drives the gate of the NFET open-drain device and pulls the voltage on the OT pin from logic 1 to logic 0 under these conditions (output trips). Furthermore, the logic 1 output from the comparator causes the hysteresis control to increase the voltage of the positive tempco reference by an amount set by the logic setting on the HYST pin (10 °C for logic 1 on the HYST pin; 2°C for logic 0 on the HYST pin). Increasing the voltage of the positive tempco reference after the TMP709 trips stops the TMP709 from untripping (voltage on the OT pin changing from logic 0 to logic 1) until the local temperature has been reduced by the amount set by the HYST pin. After the local temperature has been reduced and the voltage from the positive tempco reference is less than the voltage from the negative tempco reference, the output of the comparator switches from logic 1 to logic 0. This condition causes the voltage on the OT pin to change from logic 0 to logic 1 (device untrips). 6 Submit Documentation Feedback Copyright © 2011–2012, Texas Instruments Incorporated Product Folder Link(s): TMP709 |
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