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AD22105AR Datenblatt(PDF) 7 Page - Analog Devices |
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AD22105AR Datenblatt(HTML) 7 Page - Analog Devices |
7 / 8 page AD22105 REV. 0 –7– OUTPUT SECTION The output of the AD22105 is the collector of an NPN transistor. When the ambient temperature of the device exceeds the programmed setpoint temperature, this transistor is activated causing its collector to become a low impedance. A pull-up resistor, such as the internal 200 k Ω provided, is needed to observe a change in the output voltage. For versatility, the optional pull-up resistor has not been permanently connected to the output pin. Instead, this resistor is undedicated and connects from Pin 7 (VS) to Pin 1 (RPULL-UP). In order to use RPULL-UP a single connection should be made from Pin 1 (RPULL-UP) to Pin 2 (OUT). The 200 k Ω pull-up resistor can drive CMOS loads since essentially no static current is required at these inputs. When driving “LS” and other bipolar family logic inputs a parallel resistor may be necessary to supply the 20 µA–50 µA I IH (High Level Input Current) specified for such devices. To determine the current required, the appropriate manufacturer’s data sheet should be consulted. When the output is switched, indicating an over temperature condition, the output is capable of pulling down with 10 mA at a voltage of about 375 mV. This allows for a fan out of 2 with standard bipolar logic and 20 with “LS” family logic. Low power indicator LEDs (up to 10 mA) can be driven directly from the output pin of the AD22105. In most cases a small series resistor (usually of several hundred ohms) will be required to limit the current to the LED and the output transistor of the AD22105. MOUNTING CONSIDERATIONS If the AD22105 is thermally attached and properly protected, it can be used in any measuring situation where the maximum range of temperatures encountered is between –40 °C and +150 °C. Because plastic IC packaging technology is employed, excessive mechanical stress must be avoided when fastening the device with a clamp or screw-on heat tab. Thermally conductive epoxy or glue is recommended for typical mounting conditions. In wet or corrosive environments, an electrically isolated metal or ceramic well should be used to protect the AD22105. THERMAL ENVIRONMENT EFFECTS The thermal environment in which the AD22105 is used determines two performance traits: the effect of self-heating on accuracy and the response time of the sensor to rapid changes in temperature. In the first case, a rise in the IC junction tempera- ture above the ambient temperature is a function of two variables: the power consumption of the AD22105 and the thermal resistance between the chip and the ambient environment, θ JA. Self-heating error can be derived by multiplying the power dissipation by θ JA. Because errors of this type can vary widely for surroundings with different heat sinking capacities, it is necessary to specify θ JA under several conditions. Table I shows how the magnitude of self-heating error varies relative to the environment. A typical part will dissipate about 230 µW at room temperature with a 3.3 V supply and negligible output loading. In still air, without a “heat sink,” Table I indicates a θ JA of 190 °C/W, which yields a temperature rise of 0.04°C. Thermal rise of the die will be considerably less in an environ- ment of turbulent or constant moving air or if the device is in direct physical contact with a solid (or liquid) body. Response of the AD22105 internal die temperature to abrupt changes in ambient temperatures can be modeled by a single time constant exponential function. Figure 11 shows typical response plots for moving and still air. The time constant, τ (time to reach 63.2% of the final value), is dependent on θ JA and the thermal capacities of the chip and the package. Table I lists the effective τ for moving and still air. Copper printed circuit board connections were neglected in the analysis; however, they will sink or conduct heat directly through the AD22105’s solder plated copper leads. When faster response is required, a therm- ally conductive grease or glue between the AD22105 and the surface temperature being measured should be used. Table I. Thermal Resistance (SO-8) Medium JA ( C/Watt) (sec)* Moving Air** 100 3.5 Without Heat Sink Still Air 190 15 Without Heat Sink NOTES **The time constant is defined as the time to reach 63.2% of the final tempera- ture change. **1200 CFM. USING THE AD22105 AS A COOLING SETPOINT DETECTOR The AD22105 can be used to detect transitions from higher temperatures to lower temperatures by programming the setpoint temperature 4 °C greater than the desired trip point temperature. The 4 °C is necessary to compensate for the nominal hysteresis value designed into the device. A more precise value of the hysteresis can be obtained from Figure 6. In this mode, the logic state of the output will indicate a HIGH for under temperature conditions. The total device error will be slightly greater than the specification value due to uncertainty in hysteresis. APPLICATION HINTS EMI Suppression Noisy environments may couple electromagnetic energy into the RSET node causing the AD22105 to falsely trip or untrip. Noise sources, which typically come from fast rising edges, can be coupled into the device capacitively. Furthermore, if the output signal is brought close the RSET pin, energy can couple from the OUT pin to the RSET pin potentially causing oscillation. Stray capacitance can come from several places such as, IC sockets, multiconductor cables, and printed circuit board traces. In some cases, it can be corrected by constructing a Faraday shield around the RSET pin, for example, by using a shielded cable with the shield grounded. However, for best performance, cables should be avoided and the AD22105 should be soldered directly to a printed circuit board whenever possible. Figure 13 shows a sample printed circuit board layout with low inter-pin capaci- tance and Faraday shielding. If stray capacitance is unavoidable, and interference or oscillation occurs, a low impedance capaci- tor should be connected from the RSET pin to the GND pin. This capacitor must be considerably larger than the estimated stray capacitance. Typically several hundred picofarads will cor- rect the problem. |
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