REF19x Series
+VS
VS > VOUT2 +0.15V
C1
0.1F
U1
REF192
VO (U1)
C2
1F
+VOUT2
R1
3.735V
4.99k⍀
(SEE TEXT)
VIN
COMMON
D1
AD589
VO (D1)
C3
1F
+VOUT1
1.235V
VOUT
COMMON
Figure 20. Stacking Voltage References with the REF19x
A Precision Current Source
Many times, in low power applications, the need arises for a
precision current source that can operate on low supply volt-
ages. As shown in Figure 21, any one of the devices in the
REF19x family of references can be configured as a precision
current source. The circuit configuration illustrated is a floating
current source with a grounded load. The reference’s output
voltage is bootstrapped across RSET, which sets the output cur-
rent into the load. With this configuration, circuit precision is
maintained for load currents in the range from the reference’s
supply current (typically, 30 µA) to approximately 30 mA. The
low dropout voltage of these devices maximizes the current
source’s output voltage compliance without excess headroom.
VIN
VIN
REF19x
SLEEP
VREF
GND
1F
ISY
ADJUST
R1
RSET
P1
IOUT
VIN IOUT • RL (MAX) + VSY (MIN)
IOUT
=
VOUT
RSET
+ ISY (REF19x)
RL
VOUT
RSET
>> ISY
E.G. REF195 : VOUT = 5V
IOUT = 5mA
R1 = 953⍀
P1 = 100⍀, 10-TURN
Figure 21. A Low Dropout, Precision Current Source
The circuit’s governing equations are:
V IN = IOUT × RL(max )+V SY (min, REF19x)
IOUT
= V OUT
RSET
+ ISY (REF19x)
V OUT
RSET
〉〉ISY
(
REF19x
)
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 22 illustrates the general technique, which
takes advantage of the output “wire-OR” capability of the
REF19x device family. When OFF, a REF19x device is effec-
tively 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.
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 VC logic
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 consis-
tent with the grade of individual U1 and U2 devices.
+VS = 6V
VC 1
23
4
U3A
U3B
74HC04 74HC04
U1
REF19x
(SEE TABLE)
OUTPUT TABLE
U1/U2 VC* VOUT (V)
REF195/ HI 5.0
REF196 LO 3.3
REF194/ HI 4.5
REF195 LO 5.0
* CMOS LOGIC LEVELS
+VOUT
VIN
COMMON
C1
0.1F
U2
REF19x
(SEE TABLE)
C2
1F
VOUT
COMMON
Figure 22. Switched Output Reference
–20–
REV. D