REF19X Datasheet, PDF (23/28 Pages) Analog Devices – Precision Micropower, Low Dropout Voltage References

REF19x Series
In applications where these devices are configured as low
dropout voltage regulators, these wiring voltage drops can
become a large source of error. To circumvent this problem,
force and sense connections can be made to the reference
through the use of an operational amplifier, as shown in Figure 29.
This method provides a means by which the effects of wiring
resistance voltage drops can be eliminated. Load currents flowing
through wiring resistance produce an I-R error (ILOAD × RWIRE) at
the load. However, the Kelvin connection overcomes the problem
by including the wiring resistance within the forcing loop of the
op amp. Because the op amp senses the load voltage, op amp
loop control forces the output to compensate for the wiring
error and to produce the correct voltage at the load. Depending
on the reference device chosen, operational amplifiers that can
be used in this application are the OP295, OP292, and OP183.
SLEEP
V_IN
2
V_IN
3
V_OUT 6
REF19x
GND
4
V_IN
R_LW
2
A1
3
1 ^RLW
+V_OUT
SENSE
+V_OUT
FORCE
A1 = 1/2 OP295
1µF 100kΩ
1/2 OP292 R_L
OP183
Figure 29. A 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
configuration 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.
+V_BAT
+V_S
R1
1.1MΩ ^R3
10MΩ
R6
100Ω
V_S, V_BAT
COMMON
3+ ₇
6
2 – ⁴ U3
AD820
R2
100kΩ
C4 R5
0.1µF 100kΩ
R4
900kΩ
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,
since a constant 5 V reference voltage is desired for all
conditions, 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, VS. VBAT is a 6 V battery backup
source that supplies power to the load only when VS fails. For
normal (VS present) power conditions, VBAT sees only the 15 μA
(maximum) standby current drain of U1 in its off state.
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 VS, which
switches U3 and U1/U2, dependent upon the absolute level of
VS. 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 VS ripple. Furthermore, the
device chosen is the AD820, a rail-to-rail output device. This
device provides HI and LO output states within a few mV of VS,
ground for accurate thresholds, and compatible drive for U2 for
all VS conditions. R3 provides positive feedback for circuit
hysteresis, changing the threshold at the positive input as
a function of the output of U3.
C2
0.1µF
Q1
2N3904
2
U1
3 REF195 6
(SEE TABLE)
4
C1
0.1µF
2
U2
3 REF195 6
(SEE TABLE)
4
5.000V
+ C3
1µF
V_OUT
COMMON
Figure 30. A Fail-Safe 5 V Reference
Rev. I | Page 23 of 28