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AD1580BRT bảng dữ liệu(PDF) 6 Page - Analog Devices |
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6 / 12 page AD1580 Rev. F | Page 6 of 12 THEORY OF OPERATION The AD1580 uses the band gap concept to produce a stable, low temperature coefficient voltage reference suitable for high accuracy data acquisition components and systems. The device makes use of the underlying physical nature of a silicon tran- sistor base emitter voltage in the forward biased operating region. All such transistors have an approximately −2 mV/°C temperature coefficient (TC), which is unsuitable for use directly as a low TC reference; however, extrapolation of the temperature characteristic of any one of these devices to absolute zero (with collector current proportional to absolute temperature) reveals that its VBE goes to approximately the silicon band gap voltage. Thus, if a voltage could be developed with an opposing temperature coefficient to sum with VBE, a zero TC reference would result. The AD1580 circuit in Figure 10 provides such a compensating voltage, V1, by driving two transistors at different current densities and amplifying the resultant VBE difference (∆VBE, which has a positive TC). The sum of VBE and V1 provides a stable voltage reference. V+ V– V1 ΔVBE VBE Figure 10. Schematic Diagram APPLYING THE AD1580 The AD1580 is simple to use in virtually all applications. To operate the AD1580 as a conventional shunt regulator (see Figure 11), an external series resistor is connected between the supply voltage and the AD1580. For a given supply voltage, the series resistor, RS, determines the reverse current flowing through the AD1580. The value of RS must be chosen to accommodate the expected variations of the supply voltage, VS; load current, IL; and the AD1580 reverse voltage, VR; while maintaining an acceptable reverse current, IR, through the AD1580. The minimum value for RS should be chosen when VS is at its minimum and IL and VR are at their maximum, while maintaining the minimum acceptable reverse current. The value of RS should be large enough to limit IR to 10 mA when VS is at its maximum and IL and VR are at their minimum. The equation for selecting RS is as follows: RS = (VS − VR)/(IR + IL) Figure 12 shows a typical connection of the AD1580BRT operating at a minimum of 100 µA. This connection can provide ±1 mA to the load while accommodating ±10% power supply variations. VS IR + IL RS VOUT IL VR IR Figure 11. Typical Connection Diagram +5V(+3V) ±10% 2.94k Ω (1.30k Ω) RS VR VOUT Figure 12. Typical Connection Diagram TEMPERATURE PERFORMANCE The AD1580 is designed for reference applications where stable temperature performance is important. Extensive temperature testing and characterization ensure that the device’s performance is maintained over the specified temperature range. Some confusion exists in the area of defining and specifying reference voltage error over temperature. Historically, references have been characterized using a maximum deviation per degree Celsius, for example, 50 ppm/°C. However, because of nonlinear- ities in temperature characteristics that originated in standard Zener references (such as S type characteristics), most manufac- turers now use a maximum limit error band approach to specify devices. This technique involves the measurement of the output at three or more different temperatures to guarantee that the voltage falls within the given error band. The proprietary curvature correction design techniques used to minimize the AD1580 nonlinearities allow the temperature performance to be guaranteed using the maximum deviation method. This method is of more use to a designer than the one that simply guarantees the maximum error band over the entire temper- ature change. Figure 13 shows a typical output voltage drift for the AD1580 and illustrates the methodology. The maximum slope of the two diagonals drawn from the initial output value at +25°C to the output values at +85°C and −40°C determines the performance grade of the device. For a given grade of the AD1580, the designer can easily determine the maximum total error from the initial tolerance plus temperature variation. |
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