REF193 AD [Analog Devices], REF193 Datasheet - Page 20

• Manufacturer Part Number: REF193
• Description: Precision Micropower, Low Dropout Voltage References
• Manufacturer: AD [Analog Devices]

REF19x Series
The circuit’s governing equations 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 relatively
little additional hardware.
The circuit of Figure 9 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). As a result, for
two devices wired together at their common outputs, the output
voltage is simply that of the ON device. The OFF state device
will draw a small standby current of 15 µA (max), but otherwise
will 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.
Figure 8. A Low Dropout, Precision Current Source
V
V
V
I
OUT
I
R
IN
OUT
OUT
IN
SET
=
I
= I
SLEEP
V
V
OUT
R
R
=
〉〉I
OUT
OUT
SET
SET
V
OUT
R
SY
REF19x
OUT
SET
>> I
+ I
R
GND
V
V
L
(REF19x)
IN
IN
SY
× R
SY
(MAX) + V
+ I
(REF19x)
V
L
REF
(max)+V
SY
(REF19x)
SY
(MIN)
E.G. REF195 : V
1 F
ADJUST
SY
I
R1 = 953
P1 = 100 , 10-TURN
OUT
(min, REF19x)
I
SY
= 5mA
R
L
I
R1
P1
OUT
OUT
= 5V
R
SET
–20–
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, changing the CMOS compatible V
control voltage from HI to LO selects between a nominal output
of 5.000 V and 3.300 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 table, again). Of course, the exact output voltage tolerance,
drift, and overall quality of the reference voltage will be consistent
with the grade of individual U1 and U2 devices.
Because of the nature of the wire–OR, there is one application
caveat that 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, will neces-
sarily 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 will take
longer. Exactly how much longer will be a function of the load
resistance, R
C2. In general, a conservative transition time here will be on the
order of 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).
COMMON
+V
S
= 6V
V
V
IN
C
1
74HC04
Figure 9. Switched Output Reference
U3A
L
, seen at the output and the typical 1 µF value of
2
3
74HC04
U3B
0.1 F
C1
4
(SEE TABLE)
(SEE TABLE)
REF19x
REF19x
U1
U2
*CMOS LOGIC LEVELS
U1/U2
REF195/
REF196
REF194/
REF195
OUTPUT TABLE
HI
LO
HI
LO
V
C
C2
1 F
*
V
5.0
3.3
4.5
5.0
COMMON
+V
V
OUT
OUT
C
REV. E
OUT
logic
(V)