REF193 AD [Analog Devices], REF193 Datasheet - Page 19

• Manufacturer Part Number: REF193
• Description: Precision Micropower, Low Dropout Voltage References
• Manufacturer: AD [Analog Devices]

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
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 kΩ 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.
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.
Two reference ICs are used, fed from a common unregulated
input, V
nected in series as shown, which provides two output voltages,
V
REV. E
V
TTL/CMOS
S
COMMON
OUT1
Figure 6. Stacking Voltage References with the REF19x
> V
SLEEP
OUT2
+V
Figure 5. A Negative Precision Voltage Reference
Uses No Precision Resistors
V
and V
IN
S
S
+0.15V
. The outputs of the individual ICs are simply con-
10k
10k
OUT2
0.1 F
0.1 F
2N3906
C3
. V
C1
3
OUT1
SLEEP
REF19x
GND
V
V
2
IN
4
(SEE TABLE)
is the terminal voltage of U1, while
IN
OUT
(SEE TABLE)
REF19x
REF19x
V
OUT
U1
U2
100k
and GND. Thus, any desired
6
REF192/REF192
REF192/REF194
REF192/REF195
V
V
O
O
U1/U2
1k
(U2)
(U1)
1 F
A1 = 1/2 OP295,
OUTPUT TABLE
–5V
+5V
A1
1/2 OP291
C4
C2
1 F
1 F
1 F
V
OUT1
2.5
2.5
2.5
R1
3.9k
(SEE TEXT)
100
(V)
V
OUT2
+V
V
COMMON
+V
5.0
7.0
7.5
OUT
OUT1
OUT2
–V
(V)
REF
–19–
V
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 µA to 100 µA), 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 µA of current with the applicable
V
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
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
D1 and U1 respectively. Here this is 1.235 and 2.5, which pro-
vides a V
diodes such as D1, the rated minimum and maximum device
currents must be observed and the maximum load current from
V
In the case with V
bias to D1, so the maximum load current available at V
450 µA or less.
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
load. With this configuration, circuit precision is maintained for
load currents in the range from the reference’s supply current
(typically, 30 µA) to approximately 30 mA. The low dropout
voltage of these devices maximizes the current source’s output
voltage compliance without excess headroom.
OUT2
OUT1
OUT1
OUT1
Figure 7. Stacking Voltage References with the REF19x
V
S
COMMON
> V
is the sum of this voltage and the terminal voltage of U2.
can be no greater than the current set up by R1 and V
across it. Note that the two U1 and U2 reference circuits
and V
OUT2
+V
V
OUT2
IN
S
+0.15V
OUT2
0.1 F
of 3.735 V. When using two-terminal reference
S
C1
, is determined by the sum of the outputs, V
, which are the individual terminal voltages of
O(U1)
AD589
SET
equal to 2.5 V, R1 provides a 500 µA
D1
REF192
, which sets the output current into the
U1
V
V
O
O
(U1)
(D1)
REF19x Series
C2
1 F
C3
1 F
R1
4.99k
(SEE TEXT)
1.235V
+V
3.735V
+V
V
COMMON
OUT1
OUT
OUT2
OUT1
O(U1)
OUT2
is
,
.