8 /5 — AD580. THEORY OF OPERATION. OUT = VZ 1 +. = 2.5V. Z = VBE + V1. BE (Q1). …
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AD580. THEORY OF OPERATION. OUT = VZ 1 +. = 2.5V. Z = VBE + V1. BE (Q1). = V. BE + 2. VBE. R kT. 2I1 = I1 + I2. 1 = 2. = 1.205V. COM. R12. R13. Q14. Q13
link to page 5 link to page 5 link to page 5 link to page 5 link to page 5 AD580THEORY OF OPERATION+V The AD580 family (AD580, AD581, AD584, AD589) uses the INR8R7 bandgap concept to produce a stable, low temperature coef- R4 ficient voltage reference suitable for high accuracy data acqui- I ≅ V2I1OUT = VZ 1 += 2.5VR5 sition components and systems. The device makes use of the R4Q2Q1 underlying physical nature of a silicon transistor base-emitter V8AAZ = VBE + V1 voltage in the forward-biased operating region. All such tran- VR5 ∆ R1VBE (Q1)= VBER2BE + 2 ∆ VBER2 sistors have approximately a –2 mV/°C temperature coefficient, R kTJ= V11BE + 2lnRqJ unsuitable for use directly as a low TC reference. Extrapolation RR1V122 -004 2I1 = I1 + I21 = 2 ∆ VBER2= 1.205V of the temperature characteristic of any one of these devices to COM 00525-B absolute zero (with an emitter current propor-tional to the Figure 4. Basic Bandgap-Reference Regulator Circuit absolute temperature), however, reveals that it will go to a VBE of 1.205 V at 0 K, as shown in Figure 3. Thus, if a voltage could be +E developed with an opposing temperature coefficient to sum R12R13 with VBE to total 1.205 V, a 0 TC reference would result and Q14Q13 operation from a single, low voltage supply would be possible. Q4 The AD580 circuit provides such a compensating voltage, V1 in Figure 4, by driving two transistors at different current densities Q3Q7 and amplifying the resulting VBE difference (∆VBE—which now R8R7R6 has a positive TC). The sum, VZ, is then buffered and amplified Q10Q11Q12 up to 2.5 V to provide a usable reference-voltage output. Figure Q6 5 shows the schematic diagram of the AD580. Q8Q15R9Q5Q92.5V OUTR10 The AD580 operates as a 3-terminal reference, meaning that no R4C1Q2R3Q1 additional components are required for biasing or current 8AA setting. The connection diagram, Figure 6, is quite simple. R5R2R11 -005 R1–ECOM 00525-B 1.5 Figure 5. Schematic Diagram CONSTANT SUM = 1.205V1.205FOR BOTH+E)DEVICES(V4.5 ≤ V ≤ 30V1.0INLTAGEEOUTOAD580VBE VS. TEMPERATURE FOR TWO TYPICALLOADDEVICES (I α ET)0.5 -006 –EUNCTION V J 00525-B Figure 6. Connection Diagram REQUIRED COMPENSATION -003 VOLTAGE– SAME DEVICESVOLTAGE VARIATION VERSUS TEMPERATURE0 00525-B –273 ° C–200 ° C–100 ° C0 ° C100 ° C0K73K173K273K373K Some confusion exists in the area of defining and specifying TEMPERATURE reference voltage error over temperature. Historically, references Figure 3. Extrapolated Variation of Base-Emitter Voltage with Temperature (I are characterized using a maximum deviation per degree EαT), and Required Compensation, Shown for Two Different Devices Centigrade; i.e., 10 ppm/°C. However, because of the inconsistent nonlinearities in Zener references (butterfly or S type characteristics), most manufacturers use a maximum limit error band approach to characterize their references. This technique measures the output voltage at 3 to 5 different temperatures and guarantees that the output voltage deviation will fall within the guaranteed error band at these discrete temperatures. This approach, of course, makes no mention or guarantee of performance at any other temperature within the operating temperature range of the device. Rev. B | Page 5 of 8 Document Outline FEATURES GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS AD580 CHIP DIMENSIONS AND PAD LAYOUT ESD CAUTION THEORY OF OPERATION VOLTAGE VARIATION VERSUS TEMPERATURE NOISE PERFORMANCE THE AD580 AS A CURRENT LIMITER THE AD580 AS A LOW POWER, LOW VOLTAGE, PRECISION REFERENCE F OUTLINE DIMENSIONS ORDERING GUIDE