ref195gru-reel Analog Devices, Inc., ref195gru-reel Datasheet - Page 22

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

REF19x Series
The governing equations for the circuit are
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 (max), but otherwise does not interfere with operation 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.
V
I
OUT
IN
V
I
V
R
OUT
IN
3
≥ I
OUT
SET
=
SLEEP
OUT
V
R
V
R
=
OUT
SET
OUT
SET
=
Figure 27. A Low Dropout, Precision Current Source
〉〉
I
× R
REF19x
OUT
V
R
+ I
>> I
I
OUT
GND
L
SET
SY
V
V
SY
(MAX) + V
2
4
IN
IN
SY
×
(REF19x)
(
REF
R
+
V
L
I
REF
(
SY
Max
19x
SY
(
6
(MIN)
REF
)
)
+
19x
ADJUST
1µF
V
E.G., REF195: V
SY
)
I
SY
(
Min
R
L
I
OUT
,
R1
P1
REF
I
R1 = 953Ω
P1 = 100Ω, 10-TURN
OUT
R
OUT
SET
19x
= 5mA
= 5V
)
Rev. I | Page 22 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 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. Since U1 and U2 can only
source current effectively, negative going output voltage
changes, which require the sinking of current, necessarily takes
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
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 (>7.6 time constants for <1/2 LSB
error @ 10 bits, for example).
KELVIN CONNECTIONS
In many portable applications where the PC board 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 (1 oz. Cu, for example).
COMMON
+V
S
= 6V
V
V
IN
C
74HC04
1
U3A
C
2
logic control voltage from HI to LO selects
74HC04
Figure 28. Switched Output Reference
3
0.1µF
U3B
C1
4
3
3
(SEE TABLE)
(SEE TABLE)
REF19x
REF19x
U1
U2
2
4
2
4
6
6
U1/U2
REF195/
REF196
REF194/
REF195
* CMOS LOGIC LEVELS
OUTPUT TABLE
V
HI
LO
HI
LO
+
C
C2
1µF
*
V
5.0
3.3
4.5
5.0
+V
V
COMMON
OUT
OUT
OUT
L
, seen at
(V)