Datasheet ADR1399 (Analog Devices) - 9

Data Sheet
ADR1399 THEORY OF OPERATION OPERATING SET TEMPERATURE
overall performance with external thermocouples and/or IR drops. For example, the heater current can be quite high; therefore, avoid The ADR1399 contains a buried zener diode with an approximate sharing the heater current path with the Zener sense path. In +2 mV/℃ temperature coefficient, in series with an NPN, base addition, wherever metallurgic junctions are formed, such as where emitter voltage (VBE) with an approximate –2 mV/℃ temperature the device pins enter the board or where the reference voltage coefficient. The combined positive and negative temperature coeffi- may be connectorized, try to ensure that junctions are paired and cients sum to a nominal 0 mV/℃ overall temperature coefficient. with similar thermal gradients. Parasitic thermocouples can simply There are two op-amp servo loops inside the ADR1399. One add temperature dependencies from 1 μV/℃ up to 40 μV/℃. See op-amp loop maintains a fixed ratio of Zener and VBE currents, Application Note 86 for additional information on thermoelectric with the total current set by an externally applied pull-up resistor or potentials. current source. Another op-amp loop maintains the device die at a nonadjustable set temperature of approximately 95℃, precluding
THERMAL RESISTANCE
the external ambient temperature fluctuations from affecting operat- ing temperature. The entire system is provided in a simple 4-pin, The ADR1399 has an on-chip automated heater set to approxi- hermetically sealed, TO-46 package and placed inside a plastic mately 95°C. The ADR1399 comes from the factory provided with thermal insulator, which further keeps ambient fluctuations at bay a small plastic jacket to avoid wasting excess power in the heater and reduces the required heater power. If the ambient temperature and to help keep external temperature fluctuations further from exceeds the set temperature, the chip temperature control becomes the reference. The plastic jacket raises the effective net thermal open loop, and the temperature rejection of the device degrades resistance. Other techniques to increase thermal resistance include while the excess ambient temperature condition persists. reducing solid copper planes in proximity to the device and elevat- ing the device on its leads, approximately 1 cm above the board With only an external supply and a pull-up resistor required for surface. A mesh ground on the bottom side of the board increases operation, the ADR1399 is simple to use. However, because it thermal resistance compared to a solid ground plane. is so extremely stable, care must be taken to avoid degrading
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Document Outline Features Applications Pin Configuration General Description Specifications Electrical Characteristics Absolute Maximum Ratings Thermal Resistance ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Theory of Operation Operating Set Temperature Thermal Resistance Applications Information Avoiding Thermocouple Errors Shunt Dynamic Impedance and Capacitive Load Typical Applications Outline Dimensions Ordering Guide