AD1580 Analog Devices, AD1580 Datasheet - Page 4

• Manufacturer Part Number: AD1580
• Description: 1.2 V Micropower, Precision Shunt Voltage Reference
• Manufacturer: Analog Devices

AD1580
THEORY OF OPERATION
The AD1580 uses the “bandgap” 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
transistor base-emitter voltage in the forward-biased operating
region. All such transistors have approximately a –2 mV/ C
temperature coefficient, 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
V
if a voltage could be developed with an opposing temperature
coefficient to sum with V
The AD1580 circuit in Figure 6, provides such a compensating
voltage, V1 by driving two transistors at different current
densities and amplifying the resultant V
which has a positive TC). The sum of V
stable voltage reference.
APPLYING THE AD1580
The AD1580 is simple to use in virtually all applications. To
operate the AD1580 as a conventional shunt regulator (Figure
7a), an external series resistor is connected between the supply
voltage and the AD1580. For a given supply voltage the series
resistor, R
AD1580. The value of R
expected variations of the supply voltage, V
and the AD1580 reverse voltage, V
acceptable reverse current, I
The minimum value for R
its minimum, and I
maintaining the minimum acceptable reverse current.
The value of R
when V
The equation for selecting R
Figure 7b shows a typical connection with the AD1580BRT
operating at a minimum of 100 A that can provide 1 mA to
its load, while accommodating 10% power supply variations.
BE
will go to approximately the silicon bandgap voltage. Thus,
V
BE
V1
S
is at its maximum, and I
S
, determines the reverse current flowing through the
V
BE
S
Figure 6. Schematic Diagram
should be large enough to limit I
L
R
and V
S
= (V
BE
S
S
must be chosen to accommodate the
, a zero TC reference would result.
should be chosen when V
R
S
R
S
, through the AD1580.
– V
are at their maximum while
is as follows:
L
R
and V
)/(I
R
, while maintaining an
R
R
+ I
BE
BE
are at their minimum.
L
difference ( V
and V1 provide a
S
)
, load current, I
R
to 10 mA
S
is at
BE
V+
V–
—
L
,
–4–
TEMPERATURE PERFORMANCE
The AD1580 is designed for reference applications where
stable temperature performance is important. Extensive
temperature testing and characterization ensures 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
centigrade, i.e., 50 ppm/ C. However, because of nonlinearities
in temperature characteristics which originated in standard
Zener references (such as “S” type characteristics), most
manufacturers 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 will fall 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
which simply guarantees the maximum error band over the
entire temperature change.
Figure 8 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. For example, the
AD1580BRT initial tolerance is 1 mV, a 50 ppm/ C
temperature coefficient corresponds to an error band of 4 mV
1.2258
1.2256
1.2254
1.2252
1.2250
1.2248
1.2246
1.2244
1.2242
1.2240
1.2238
Figure 8. Output Voltage vs. Temperature
Figure 7. Typical Connection Diagram
–55
I
R
SLOPE = TC = ———————————––––
R
V
–35
S
R
V
V
S
MIN
(a)
I
–15
R +
I
L
I
SLOPE = TC = ———————————–––––
L
(85 C – 25 C) x 1.225 x 10
V
OUT
5
TEMPERATURE – C
(V
25
MAX
V
+5V(+3V) 10%
O
(–40 C – 25 C) x 1.225 x 10
– V
R
45
V
S
O
R
)
(V
2.94k
(1.30k )
(b)
65
MIN
–6
V
– V
MAX
85
V
O
OUT
)
105
–6
125
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