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AD694AQ Fiches technique(PDF) 7 Page - Analog Devices |
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AD694AQ Fiches technique(HTML) 7 Page - Analog Devices |
7 / 16 page AD694 REV. B –7– Figure 4. Using Optional Pass Transistor to Minimize Self-Heating Errors; Dual Supply Operation Shown POWER DISSIPATION CONSIDERATIONS The AD694 is rated for operation over its specified temperature without the use of an external pass transistor. However, it is possible to exceed the absolute maximum power dissipation, with some combinations of power supply voltage and voltage reference load. The internal dissipation of the part can be calcu- lated to determine if there is a chance that the absolute maxi- mum dissipation may be exceeded. The die temperature must never exceed 150 °C. Total power dissipation (PTOT), is the sum of power dissipated by the internal amplifiers, P (Standing), the voltage reference, P(VREF) and the current output stage, P(IOUT) as follows: PTOT = P (Standing) + P (VREF) + P (IOUT) where: P (Standing) = 2 mA (max) × VS P (VREF) = (VS – VREF) × IVREF P(IOUT) (VS – VOUT) × I OUT (max): IOUT (max) may be the max expected operating cur- rent, or the overdriven current of the device. P(IOUT) drops to (2 V × IOUT) if a pass transistor is used. Definitions: VREF = output voltage of reference IVREF = output current of reference VS = supply voltage VOUT = voltage at IOUT pin. An appropriate safety factor should be added to PTOT. The junction temperature may be calculated with the following formula: TJ = PTOT ( θ JC + θ CA) + TAMBIENT θJC is the thermal resistance between the chip and the package (case), θCA is the thermal resistance between the case and its surroundings and is determined by the characteristics of the thermal connection of the case to ambient. For example, assume that the part is operating with a VS of 24 V in the CERDIP package at 50 °C, with a 1 mA load on the 10 V reference. Assume that IOUT is grounded and that the max IOUT would be 20 mA. The internal dissipation would be: P(TOT) = 2 mA × 24 V + (24 V – 10 V) × 1 mA + (24 V – 0 V) × 20 mA = 48 mW + 14 mW + 480 mW = 542 mW Using θJC of 30°C/W and θCA of 70°C/W (from specifications page), the junction temperature is: TJ = 542 mW (30 °C/W + 70°C/W) + 50°C = 104.2°C The junction temperature is in the safe region. Internal power dissipation can be reduced either by reducing the value of θ CA through the use of air flow or heat sinks, or by re- ducing PTOT of the AD694 through the use of an external pass transistor. Figure 5 shows the maximum case and still air tem- peratures for a given level of power dissipation. Figure 5. Internal Power Dissipation in mW ADJUSTMENT PROCEDURES The following sections describe methods for trimming the out- put current offset, the span, and the voltage reference. ADJUSTING 4 mA ZERO The 4 mA zero current may be adjusted over the range of 2 mA to 4.8 mA to accommodate large input signal offsets, or to allow small adjustment in the zero current. The zero may be adjusted by pulling up or down on Pin 6 (4 mA Adj) to increase or de- crease the nominal offset current. The 4 mA Adj. (Pin 6) should not be driven to a voltage greater than 1 V. The arrangement of |
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