Datasheet

REF19x Series
Rev. J | Page 22 of 28
One caveat to this approach is that 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; consider this issue when choosing the
negative supply for the circuit.
100Ω
1µF
1kΩ
1µF
–V
REF
REF19x
V
S
GND
OUTPUT
100kΩ
SLEEP
TTL/CMOS
A1 = 1/2 OP295,
1/2 OP291
V
S
10kΩ
2N3906
3 6
2
4
SLEEP
10kΩ
+5V
–5V
A1
00371-024
Figure 24. 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 shown
in Figure 25 shows how this stacked output reference can be
implemented.
R1
3.9kΩ
(SEE TEXT)
C1
0.1µF
+V
S
V
S
> V
OUT2
+ 0.15V
V
IN
COMMON
V
OUT
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
C2
1µF
C3
0.1µF
+V
OUT1
C4
1µF
U2
REF19x
(SEE TABLE)
2
63
4
U1
REF19x
(SEE TABLE)
2
63
4
+
+
V
O
(U2)
V
O
(U1)
00371-025
Figure 25. 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 connected in
series, as shown in Figure 25, which provide two output
voltages, V
OUT1
and V
OUT2
. V
OUT1
is the terminal voltage of U1,
whereas V
OUT2
is the sum of this voltage and the terminal
voltage of U2. U1 and U2 are chosen for the two voltages that
supply the required outputs (see Table 1). If, for example, both
U1 and U2 are REF192s, the two outputs are 2.5 V and 5.0 V.
Although this concept is simple, some cautions are needed.
Because 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 conservatively passes 600 μA 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 3-terminal references is
shown in Figure 26, where U1, a REF192, is stacked with a
2-terminal reference diode, such as the AD589. Like the
3-terminal stacked reference shown in Figure 25, this cir-
cuit provides two outputs, V
OUT1
and V
OUT2
, which are the
individual terminal voltages of D1 and U1, respectively. Here
this is 1.235 V and 2.5 V, which provides a V
OUT2
of 3.735 V.
When using 2-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 setup by R1 and V
O
(U1). When V
O
(U1) is
equal to 2.5 V, R1 provides a 500 μA bias to D1, so the maxi-
mum load current available at V
OUT1
is 450 μA or less.
D1
AD589
R1
4.99kΩ
(SEE TEXT)
C1
0.1μF
+V
S
V
S
> V
OUT2
+ 0.15V
V
IN
COMMON
V
OUT
COMMON
+V
OUT2
3.735V
C2
1μF
+V
OUT1
1.235V
C3
1μF
U1
REF192
2
63
4
+
+
V
O
(U1)
V
O
(D1)
00371-026
Figure 26. Stacking Voltage References with the REF192
PRECISION CURRENT SOURCE
In low power applications, the need often arises for a precision
current source that can operate on low supply voltages. As
shown in Figure 27, 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 output voltage of the reference 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 μA) to approximately 30 mA. The low dropout
voltage of these devices maximizes the current source’s output
voltage compliance without excess headroom.