Datasheet AD580 (Analog Devices) - 5

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AD580 THEORY OF OPERATION +V
The AD580 family (AD580, AD581, AD584, AD589) uses the
IN R8 R7
bandgap concept to produce a stable, low temperature coef-
R4
ficient voltage reference suitable for high accuracy data acqui-
I
≅
V 2 I1 OUT = VZ 1 + = 2.5V R5
sition components and systems. The device makes use of the
R4 Q2 Q1
underlying physical nature of a silicon transistor base-emitter
V 8A A Z = VBE + V1
voltage in the forward-biased operating region. All such tran-
V R5
∆
R1 V BE (Q1) = V BE R2 BE + 2
∆
VBE R2
sistors have approximately a –2 mV/°C temperature coefficient,
R kT J = V 1 1 BE + 2 ln R q J
unsuitable for use directly as a low TC reference. Extrapolation
R R1 V 1 2 2
-004
2I1 = I1 + I2 1 = 2
∆
VBE R2 = 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
R12 R13
with VBE to total 1.205 V, a 0 TC reference would result and
Q14 Q13
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
Q3 Q7
and amplifying the resulting VBE difference (∆VBE—which now
R8 R7 R6
has a positive TC). The sum, VZ, is then buffered and amplified
Q10 Q11 Q12
up to 2.5 V to provide a usable reference-voltage output. Figure
Q6
5 shows the schematic diagram of the AD580.
Q8 Q15 R9 Q5 Q9 2.5V OUT R10
The AD580 operates as a 3-terminal reference, meaning that no
R4 C1 Q2 R3 Q1
additional components are required for biasing or current
8A A
setting. The connection diagram, Figure 6, is quite simple.
R5 R2 R11
-005
R1 –E COM
00525-B
1.5
Figure 5. Schematic Diagram
CONSTANT SUM = 1.205V 1.205 FOR BOTH +E ) DEVICES (V 4.5
≤
V
≤
30V 1.0 IN LTAGE EOUT O AD580 VBE VS. TEMPERATURE FOR TWO TYPICAL LOAD DEVICES (I
α
E T) 0.5
-006
–E UNCTION V J
00525-B Figure 6. Connection Diagram
REQUIRED COMPENSATION
-003
VOLTAGE– SAME DEVICES VOLTAGE VARIATION VERSUS TEMPERATURE 0
00525-B
–273
°
C –200
°
C –100
°
C 0
°
C 100
°
C 0K 73K 173K 273K 373K
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