Datasheet AD1580 (Analog Devices) - 6

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AD1580 THEORY OF OPERATION
The AD1580 uses the band gap concept to produce a stable, Figure 12 shows a typical connection of the AD1580BRT low temperature coefficient voltage reference suitable for high operating at a minimum of 100 µA. This connection can accuracy data acquisition components and systems. The device provide ±1 mA to the load while accommodating ±10% makes use of the underlying physical nature of a silicon tran- power supply variations. sistor base emitter voltage in the forward biased operating
VS
region. All such transistors have an approximately −2 mV/°C temperature coefficient (TC), which is unsuitable for use
R I S R + IL
directly as a low TC reference; however, extrapolation of
IL VR
the temperature characteristic of any one of these devices to
IR V
absolute zero (with collector current proportional to absolute
OUT
011 temperature) reveals that its VBE goes to approximately the silicon band gap voltage. Thus, if a voltage could be developed 00700- Figure 11. Typical Connection Diagram 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
+5V(+3V) ±10%
transistors at different current densities and amplifying the
2.94k R Ω
resultant V
S (1.30kΩ)
BE difference (∆VBE, which has a positive TC). The sum of VBE and V1 provides a stable voltage reference.
VR V+ VOUT
012 00700- Figure 12. Typical Connection Diagram
V1 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
ΔVBE
maintained over the specified temperature range. Some confusion exists in the area of defining and specifying
V
010 reference voltage error over temperature. Historically, references
BE V–
00700- have been characterized using a maximum deviation per degree Figure 10. Schematic Diagram Celsius, for example, 50 ppm/°C. However, because of nonlinear-
APPLYING THE AD1580
ities in temperature characteristics that originated in standard Zener references (such as S type characteristics), most manufac- The AD1580 is simple to use in virtually all applications. To turers now use a maximum limit error band approach to specify operate the AD1580 as a conventional shunt regulator (see devices. This technique involves the measurement of the output Figure 11), an external series resistor is connected between the at three or more different temperatures to guarantee that the supply voltage and the AD1580. For a given supply voltage, the voltage falls within the given error band. The proprietary series resistor, RS, determines the reverse current flowing through curvature correction design techniques used to minimize the the AD1580. The value of RS must be chosen to accommodate AD1580 nonlinearities allow the temperature performance to the expected variations of the supply voltage, VS; load current, be guaranteed using the maximum deviation method. This IL; and the AD1580 reverse voltage, VR; while maintaining an method is of more use to a designer than the one that simply acceptable reverse current, IR, through the AD1580. guarantees the maximum error band over the entire temper- The minimum value for RS should be chosen when VS is at ature change. its minimum and IL and VR are at their maximum, while Figure 13 shows a typical output voltage drift for the AD1580 maintaining the minimum acceptable reverse current. and illustrates the methodology. The maximum slope of the two The value of RS should be large enough to limit IR to 10 mA diagonals drawn from the initial output value at +25°C to the when VS is at its maximum and IL and VR are at their minimum. output values at +85°C and −40°C determines the performance The equation for selecting R grade of the device. For a given grade of the AD1580, the designer S is as follows: can easily determine the maximum total error from the initial RS = (VS − VR)/(IR + IL) tolerance plus temperature variation. Rev. F | Page 6 of 12 Document Outline Features Applications General Description Pin Configurations Revision History Specifications Absolute Maximum Ratings ESD Caution Typical Performance Characteristics Theory of Operation Applying the AD1580 Temperature Performance Voltage Output Nonlinearity vs. Temperature Reverse Voltage Hysteresis Output Impedance vs. Frequency Noise Performance and Reduction Turn-On Time Transient Response Precision Micropower Low Dropout Reference Using the AD1580 with 3 V Data Converters Outline Dimensions Ordering Guide Package Branding Information