Datasheet

REF19x Series
Rev. J | Page 24 of 28
2
3
1
+V
OUT
SENSE
+V
OUT
FORCE
R
LW
1µF 100kΩ
R
L
REF19x
V
S
GND
OUTPUT
A1 = 1/2 OP295
1/2 OP292
OP183
V
S
6
2
4
SLEEP
V
S
R
LW
A1
3
00371-029
In operation, it is assumed that for all conditions, either U1 or
U2 is on, and a 5 V reference output is available. With this
voltage constant, a scaled down version is applied to the
Comparator IC U3, providing a fixed 0.5 V input to the negative
input for all power conditions. The R1 to R2 divider provides a
signal to the U3 positive input proportionally to V
S
, which
switches U3 and U1/U2, dependent upon the absolute level of
V
S
. In Figure 30, Op Amp U3 is configured as a comparator
with hysteresis, which provides clean, noise-free output
switching. This hysteresis is important to eliminate rapid
switching at the threshold due to V
S
ripple. Furthermore, the
device chosen is the AD820, a rail-to-rail output device. This
device provides high and low output states within a few mV of
V
S
, ground for accurate thresholds, and compatible drive for U2
for all V
S
conditions. R3 provides positive feedback for circuit
hysteresis, changing the threshold at the positive input as a
function of the output of U3.
Figure 29. Low Dropout, Kelvin-Connected Voltage Reference
FAIL-SAFE 5 V REFERENCE
Some critical applications require a reference voltage to be
maintained at a constant voltage, even with a loss of primary
power. The low standby power of the REF19x series and the
switched output capability allow a fail-safe reference con-
figuration to be implemented rather easily. This reference
maintains a tight output voltage tolerance for either a primary
power source (ac line derived) or a standby (battery derived)
power source, automatically switching between the two as the
power conditions change.
For V
S
levels lower than the lower threshold, the output of U3 is
low, thus U2 and Q1 are off and U1 is on. For V
S
levels higher
than the upper threshold, the situation reverses, with U1 off and
both U2 and Q1 on. In the interest of battery power conserva-
tion, all of the comparison switching circuitry is powered from
V
S
and is arranged so that when V
S
fails, the default output
comes from U1.
The circuit in Figure 30 illustrates this concept, which borrows
from the switched output idea of Figure 28, again using the
REF19x device family output wire-OR capability. In this case,
because a constant 5 V reference voltage is desired for all condi-
tions, two REF195 devices are used for U1 and U2, with their
on/off switching controlled by the presence or absence of the
primary dc supply source, V
S
. V
BAT
is a 6 V battery backup
source that supplies power to the load only when V
S
fails. For
normal (V
S
present) power conditions, V
BAT
sees only the 15 μA
(maximum) standby current drain of U1 in its off state.
For the R1 to R3 values, as shown in Figure 30, lower/upper V
S
switching thresholds are approximately 5.5 V and 6 V, respec-
tively. These can be changed to suit other V
S
supplies, as can the
REF19x devices used for U1 and U2, over a range of 2.5 V to
5 V of output. U3 can operate down to a V
S
of 3.3 V, which is
generally compatible with all REF19x family devices.
R5
100kΩ
R6
100Ω
R3
10MΩ
U3
AD820
C4
0.1µF
R2
100kΩ
R1
1.1MΩ
Q1
2N3904
+V
BAT
V
S
, V
BAT
COMMON
V
OUT
COMMON
C2
0.1µF
C1
0.1µF
5.000V
C3
1µF
U1
REF195
(SEE TABLE)
2
3
4
U2
REF195
(SEE TABLE)
2
3
4
+
6
6
+V
S
R4
900kΩ
4
2
7
6
3
+
–
00371-030
Figure 30. Fail-Safe 5 V Reference