ref195gru-reel Analog Devices, Inc., ref195gru-reel Datasheet - Page 23

• Manufacturer Part Number: ref195gru-reel
• Description: Precision Micropower, Low Dropout Voltage References
• Manufacturer: Analog Devices, Inc.

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 (I
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.
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.
SLEEP
Figure 29. A Low Dropout, Kelvin-Connected Voltage Reference
3
REF19x
GND
V
V
2
4
IN
IN
V
OUT
COMMON
6
V
S
+V
, V
+V
BAT
BAT
1µF
S
100kΩ
2
3
R1
1.1MΩ
R2
100kΩ
A1
V
A1 = 1/2 OP295
C4
0.1µF
IN
1/2 OP292
OP183
10MΩ
1
R3
R5
100kΩ
R
R
3
2
LW
LW
LOAD
+
–
7
4
AD820
× R
+V
FORCE
R
+V
SENSE
L
U3
6
Figure 30. A Fail-Safe 5 V Reference
OUT
OUT
WIRE
900kΩ
100Ω
) at
R6
R4
Rev. I | Page 23 of 28
Q1
2N3904
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, V
source that supplies power to the load only when V
normal (V
(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 V
switches U3 and U1/U2, dependent upon the absolute level of
V
with hysteresis, which provides clean, noise-free output
switching. This hysteresis is important to eliminate rapid
switching at the threshold due to V
device chosen is the AD820, a rail-to-rail output device. This
device provides HI and LO output states within a few mV of V
ground for accurate thresholds, and compatible drive for U2 for
all V
hysteresis, changing the threshold at the positive input as
a function of the output of U3.
0.1µF
C2
S
. In Figure 30, Op Amp U3 is configured as a comparator
S
C1
0.1µF
conditions. R3 provides positive feedback for circuit
3
3
(SEE TABLE)
(SEE TABLE)
REF195
REF195
S
U1
U2
present) power conditions, V
2
4
2
4
6
6
+
C3
1µF
5.000V
V
COMMON
OUT
S
. V
S
BAT
ripple. Furthermore, the
is a 6 V battery backup
REF19x Series
BAT
sees only the 15 μA
S
S
fails. For
, which
S
,