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AD22105AR bảng dữ liệu(PDF) 6 Page - Analog Devices |
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AD22105AR bảng dữ liệu(HTML) 6 Page - Analog Devices |
6 / 8 page AD22105 REV. 0 –6– PRODUCT DESCRIPTION The AD22105 is a single supply semiconductor thermostat switch that utilizes a unique circuit architecture to realize the combined functions of a temperature sensor, setpoint comparator, and output stage all in one integrated circuit. By using one external resistor, the AD22105 can be programmed to switch at any temperature selected by the system designer in the range of –40 °C to +150°C. The internal comparator is designed to switch very accurately as the ambient temperature rises past the setpoint temperature. When the ambient temperature falls, the comparator relaxes its output at a somewhat lower temperature than that at which it originally switched. The difference between the “switch” and “unswitch” temperatures, known as the hysteresis, is designed to be nominally 4 °C. THE SETPOINT RESISTOR The setpoint resistor is determined by the equation: RSET = 39 M Ω°C TSET (°C)+ 281.6°C –90.3 k Ω Eq. 1 The setpoint resistor should be connected directly between the RSET pin (Pin 6) and the GND pin (Pin 3). If a ground plane is used, the resistor may be connected directly to this plane at the closest available point. The setpoint resistor, RSET, can be of nearly any resistor type, but its initial tolerance and thermal drift will affect the accuracy of the programmed switching temperature. For most applications, a 1% metal-film resistor will provide the best tradeoff between cost and accuracy. Calculations for computing an error budget can be found in the section “Effect of Resistor Tolerance and Thermal Drift on Setpoint Accuracy.” Once RSET has been calculated, it may be found that the calcu- lated value does not agree with readily available standard resistors of the chosen tolerance. In order to achieve an RSET value as close as possible to the calculated value, a compound resistor can be constructed by connecting two resistors in series or in parallel. To conserve cost, one moderately precise resistor and one lower precision resistor can be combined. If the mod- erately precise resistor provides most of the necessary resistance, the lower precision resistor can provide a fine adjustment. Con- sider an example where the closest standard 1% resistor has only 90% of the value required for RSET. If a 5% series resistor is used for the remainder, then its tolerance only adds 5% of 10% or 0.5% additional error to the combination. Likewise, the 1% resistor only contributes 90% of 1% or 0.9% error to the combi- nation. These two contributions are additive resulting in a total compound resistor tolerance of 1.4%. EFFECT OF RESISTOR TOLERANCE AND THERMAL DRIFT ON SETPOINT ACCURACY Figure 3 shows the typical accuracy error in setpoint temperature as a function of the programmed setpoint temperature. This curve assumes an ideal resistor for RSET. The graph of Figure 4 may be used to calculate additional setpoint error as a function of resistor tolerance. Note that this curve shows additional error beyond the initial accuracy error of the part and should be added to the value found in the specifications table. For example, consider using the AD22105 programmed to switch at +125 °C. Figure 4 indicates that at +125 °C, the additional error is approximately –0.2 °C/% of R SET. If a 1% resistor (of exactly correct nominal value) is chosen, then the additional error could be –0.2 °C/% × 1% or –0.2°C. If the closest standard resistor value is 0.6% away from the calculated value, then the total error would be 0.6% for the nominal value and 1% for the tolerance or (1.006) × (1.10) or 1.01606 (about 1.6%). This could lead to an additional setpoint error as high as 0.32 °C. For additional accuracy considerations, the thermal drift of the setpoint resistor can be taken into account. For example, con- sider that the drift of the metal film resistor is 100 ppm/ °C. Since this drift is usually referred to +25 °C, the setpoint resistor can be in error by an additional 100 ppm/ °C × (125°C – 25°C) or 1%. Using a setpoint temperature of 125 °C as discussed above, this error source would add an additional –0.2 °C (for positive drift) making the overall setpoint error potentially –0.52 °C higher than the original accuracy error. Initial tolerance and thermal drift effects of the setpoint resistor can be combined and calculated by using the following equation: RMAX = RNOM ×(1+ε)× 1+ TC ×(TSET –25°C) () where: RMAX is the worst case value that the setpoint resistor can be at TSET, RNOM is the standard resistor with a value closest to the desired RSET, ε is the 25 °C tolerance of the chosen resistor (usually 1%, 5%, or 10%), TC is the temperature coefficient of the available resistor, TSET is the desired setpoint temperature. Once calculated, RMAX may be compared to the desired RSET from Equation 1. Continuing the example from above, the required value of RSET at a TSET of 125 °C is 5.566 kΩ. If the nearest standard resistor value is 5.600 k Ω, then its worst case maximum value at 125 °C could be 5.713 kΩ. Again this is +2.6% higher than RSET leading to a total additional error of –0.52 °C beyond that given by the specifications table. THE HYSTERESIS AND SELF-HEATING The actual value of the hysteresis generally has a minor dependence on the programmed setpoint temperature as shown in Figure 6. Furthermore, the hysteresis can be affected by self- heating if the device is driving a heavy load. For example, if the device is driving a load of 5 mA at an output voltage (given by Figure 9) of 250 mV, then the additional power dissipation would be approximately 1.25 mW. With a θ JA of 190 °C/W in free air the internal die temperature could be 0.24 °C higher than ambient leading to an increase of 0.24 °C in hysteresis. In the presence of a heat sink or turbulent environment, the additional hysteresis will be less. |
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