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參數(shù)資料
| 型號： | REF19XES3 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Precision Micropower, Low Dropout Voltage References |
| 中文描述： | 精密微功耗，低壓差電壓基準(zhǔn) |
| 文件頁數(shù)： | 19/24頁 |
| 文件大小： | 364K |
| 代理商： | REF19XES3 |
REV. E
REF19x Series
–19–
The circuit illustrated in Figure 5 avoids the need for tightly
matched resistors with the use of an active integrator circuit. In
this circuit, the output of the voltage reference provides the input
drive for the integrator. The integrator, to maintain circuit equi-
librium, adjusts its output to establish the proper relationship
between the reference
’
s V
OUT
and GND. Thus, any desired
negative output voltage can be chosen by simply substituting for
the appropriate reference IC. The sleep feature is maintained in
the circuit with the simple addition of a PNP transistor and a
10 k
resistor. One caveat with this approach should be men-
tioned: although rail-to-rail output amplifiers work best in the
application, these operational amplifiers require a finite amount
(mV) of headroom when required to provide any load current.
The choice for the circuit
’
s negative supply should take this issue
into account.
A1
+5V
–5V
100
1 F
1k
1 F
–V
REF
100k
REF19x
V
IN
GND
4
V
OUT
SLEEP
10k
SLEEP
TTL/CMOS
A1 = 1/2 OP295,
1/2 OP291
V
IN
10k
2
3
6
2N3906
Figure 5. A Negative Precision Voltage Reference
Uses No Precision Resistors
Stacking Reference ICs for Arbitrary Outputs
Some applications may require two reference voltage sources that
are a combined sum of standard outputs. The circuit of Figure 6
shows how this
“
stacked output
”
reference can be implemented.
U2
(SEE TABLE)
R1
3.9k
(SEE TEXT)
C1
0.1 F
+V
S
V
S
> V
OUT2
+0.15V
V
IN
COMMON
V
COMMON
OUTPUT TABLE
U1/U2
REF192/REF192
REF192/REF194
REF192/REF195
V
OUT1
(V)
2.5
2.5
2.5
V
OUT2
(V)
5.0
7.0
7.5
+V
OUT2
V
O
(U2)
C2
1 F
C3
0.1 F
+V
OUT1
V
O
(U1)
C4
1 F
U1
(SEE TABLE)
Figure 6. Stacking Voltage References with the REF19x
Two reference ICs are used, fed from a common unregulated
input, V
S
. The outputs of the individual ICs are simply con-
nected in series as shown, which provides two output voltages,
V
OUT1
and V
OUT2
. V
OUT1
is the terminal voltage of U1, while
V
OUT2
is the sum of this voltage and the terminal voltage of U2.
U1 and U2 are simply chosen for the two voltages that supply
the required outputs (see Table I). If, for example, both U1 and
U2 are REF192s, the two outputs are 2.5 V and 5.0 V.
While this concept is simple, some cautions are in order. Since
the lower reference circuit must sink a small bias current from
U2 (50
μ
A to 100
μ
A), plus the base current from the series PNP
output transistor in U2, either the external load of U1 or R1
must provide a path for this current. If the U1 minimum load is
not well defined, resistor R1 should be used, set to a value that
will conservatively pass 600
μ
A of current with the applicable
V
OUT1
across it. Note that the two U1 and U2 reference circuits
are locally treated as macrocells, each having its own bypasses at
input and output for best stability. Both U1 and U2 in this circuit
can source dc currents up to their full rating. The minimum
input voltage, V
S
, is determined by the sum of the outputs, V
OUT2
,
plus the dropout voltage of U2.
A related variation on stacking two three-terminal references
is shown in Figure 6, where U1, a REF192, is stacked with a
two-terminal reference diode such as the AD589. Like the three-
terminal stacked reference above, this circuit provides two outputs,
V
OUT1
and V
OUT2
, which are the individual terminal voltages of
D1 and U1 respectively. Here this is 1.235 and 2.5, which pro-
vides a V
OUT2
of 3.735 V. When using two-terminal reference
diodes such as D1, the rated minimum and maximum device
currents must be observed and the maximum load current from
V
OUT1
can be no greater than the current set up by R1 and V
O(U1)
.
In the case with V
O(U1)
equal to 2.5 V, R1 provides a 500
μ
A
bias to D1, so the maximum load current available at V
OUT1
is
450
μ
A or less.
U1
REF192
R1
4.99k
(SEE TEXT)
C1
0.1 F
+V
S
V
S
> V
OUT2
+0.15V
V
IN
COMMON
V
COMMON
+V
3.735V
V
O
(U1)
C2
1 F
D1
AD589
V
O
(D1)
C3
1 F
+V
1.235V
Figure 7. Stacking Voltage References with the REF19x
A Precision Current Source
Many times, in low power applications, the need arises for a
precision current source that can operate on low supply voltages.
As shown in Figure 8, any one of the devices in the REF19x
family of references can be configured as a precision current
source. The circuit configuration illustrated is a floating current
source with a grounded load. The reference
’
s output voltage is
bootstrapped across R
SET
, which sets the output current into the
load. With this configuration, circuit precision is maintained for
load currents in the range from the reference
’
s supply current
(typically, 30
μ
A) to approximately 30 mA. The low dropout
voltage of these devices maximizes the current source
’
s output
voltage compliance without excess headroom.
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