ref192gru Analog Devices, Inc., ref192gru Datasheet - Page 23

• Manufacturer Part Number: ref192gru
• Description: Precision Micropower, Low Dropout Voltage References
• Manufacturer: Analog Devices, Inc.

SWITCHED OUTPUT 5 V/3.3 V REFERENCE
Applications often require digital control of reference voltages,
selecting between one stable voltage and a second. With the
sleep feature inherent to the REF19x series, switched output
reference configurations are easily implemented with little
additional hardware.
The circuit in Figure 28 illustrates the general technique, which
takes advantage of the output wire-OR capability of the REF19x
device family. When off, a REF19x device is effectively an open
circuit at the output node with respect to the power supply.
When on, a REF19x device can source current up to its current
rating, but sink only a few μA (essentially, just the relatively low
current of the internal output scaling divider). Consequently,
when two devices are wired together at their common outputs,
the output voltage is the same as the output voltage for the on
device. The off state device draws a small standby current of
15 μA (maximum), but otherwise does not interfere with oper-
ation of the on device, which can operate to its full current
rating. Note that the two devices in the circuit conveniently
share both input and output capacitors, and with CMOS logic
drive, it is power efficient.
COMMON
+V
V
I
OUT
IN
S
≥ I
= 6V
=
V
V
OUT
V
R
V
R
IN
3
C
OUT
SET
OUT
SET
SLEEP
74HC04
× R
1
U3A
Figure 27. A Low Dropout, Precision Current Source
+ I
>> I
L
REF19x
SY
(MAX) + V
2
SY
GND
(REF19x)
V
V
Figure 28. Switched Output Reference
2
4
74HC04
0.1µF
S
S
3
OUTPUT
U3B
C1
4
SY
FOR EXAMPLE, REF195: V
(MIN)
6
3
3
(SEE TABLE)
(SEE TABLE)
REF19x
REF19x
ADJUST
U1
U2
1µF
2
4
2
4
I
SY
6
6
R
I
L
* CMOS LOGIC LEVELS
OUT
U1/U2
REF195/
REF196
REF194/
REF195
R1
P1
I
R1 = 953Ω
P1 = 100Ω, 10-TURN
OUT
OUT
OUTPUT TABLE
R
SET
= 5mA
= 5V
V
HIGH
LOW
HIGH
LOW
+
C
C2
1µF
*
+V
V
COMMON
V
5.0
3.3
4.5
5.0
OUT
OUT
OUT
(V)
Rev. J | Page 23 of 28
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 28, changing the CMOS-
compatible V
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 28). 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.
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
at the output and the typical 1 μF value of C2. In general, a con-
servative 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).
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 interconnections of a circuit can exhibit a typical
line resistance of 0.45 mΩ/square (for example, 1 oz. Cu).
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 prob-
lem 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.
C
logic control voltage from high to low selects
REF19x Series
LOAD
× R
L
, seen
WIRE
) at