Datasheet

REF19x Series Data Sheet
Rev. L | 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
Using dissimilar REF19x series devices with this configuration
allows logic selection between the U1/U2-specified terminal
voltages. For example, with U1 (a REF195) and U2 (a REF196),
as noted in the table in Figure 30, changing the CMOS-compatible
V
C
logic control voltage from high to low selects between a nominal
output of 5.0 V and 3.3 V, and vice versa. Other REF19x family
units can also be used for U1/U2, with similar operation in a
logic sense, but with outputs as per the individual paired devices
(see the table in Figure 30). Of course, the exact output voltage
tolerance, drift, and overall quality of the reference voltage is
consistent with the grade of individual U1 and U2 devices.
Figure 31. 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.
Due to the nature of the wire-OR, one application caveat should
be understood about this circuit. Because U1 and U2 can only
source current effectively, negative going output voltage changes,
which require the sinking of current, necessarily take longer than
positive going changes. In practice, this means that the circuit is
quite fast when undergoing a transition from 3.3 V to 5 V, but the
transition from 5 V to 3.3 V takes longer. Exactly how much
longer is a function of the load resistance, R
L
, seen at the output and
the typical 1 μF value of C2. In general, a conservative transition
time is approximately several milliseconds for load resistances
in the range of 100 Ω to 1 kΩ. Note that for highest accuracy at
the new output voltage, several time constants should be allowed
(for example, >7.6 time constants for <1/2 LSB error @ 10 bits).
The circuit in Figure 32 illustrates this concept, which borrows
from the switched output idea of Figure 30, 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.
KELVIN CONNECTIONS
In many portable applications where the PCB cost and area go
hand-in-hand, circuit interconnects are very often narrow. These
narrow lines can cause large voltage drops if the voltage reference is
required to provide load currents to various functions. The inter-
connections of a circuit can exhibit a typical line resistance of
0.45 mΩ/square (for example, 1 oz. Cu).
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 32, 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.
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 31. 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
LOAD
× R
WIRE
) 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.