OP191 Datasheet, PDF (15/20 Pages) Analog Devices – Micropower Single-Supply Rail-to-Rail Input/Output Op Amps

A +2.5 V Reference from a +3 V Supply
In many single-supply applications, the need for a 2.5 V
reference often arises. Many commercially available monolithic
2.5 V references require at least a minimum operating supply
voltage of 4 V. The problem is exacerbated when the minimum
operating system supply voltage is + 3 V. The circuit illustrated
in Figure 61 is an example of a +2.5 V that operates from a
single +3 V supply. The circuit takes advantage of the OP291’s
rail-to-rail input and output voltage ranges to amplify an
AD589’s 1.235 V output to +2.5 V. The OP291’s low TCV_OS
of 1 µV/°C helps to maintain an output voltage temperature
coefficient of less than 200 ppm/°C. The circuit’s overall
temperature coefficient is dominated by R2 and R3’s tempera-
ture coefficient. Lower tempco resistors are recommended.
The entire circuit draws less than 420 µA from a +3 V supply
at +25°C.
R1
17.4kΩ
AD589
+3V
3
8
1/2
1
2 ^OP291
4
R3
100kΩ
R2
P1
100kΩ 5kΩ
+2.5V_REF
RESISTORS = 1%, 100ppm/°C
POTENTIOMETER = 10 TURN, 100ppm/°C
Figure 61. A +2.5 V Reference that Operates on a Single
+3 V Supply
+5 V Only, 12-Bit DAC Swings Rail-to-Rail
The OP191 family is ideal for use with a CMOS DAC to
generate a digitally controlled voltage with a wide output range.
Figure 62 shows the DAC8043 used in conjunction with the
AD589 to generate a voltage output from 0 V to 1.23 V The
DAC is actually operated in “voltage switching” mode where
the reference is connected to the current output, I_OUT, and the
output voltage is taken from the V_REF pin. This topology is
inherently noninverting as opposed to the classic current output
mode, which is inverting and, therefore, unsuitable for single
supply.
+5V
R1
17.8kΩ
1.23V
AD589
8
V_DD
2
R_FB
3
I_OUT
DAC-8043 _VREF ¹
+5V
GND CLK SR1 LD
4 7 65
3
8
1/2
1
DIGITAL
2 ^OP291
CONTROL
4
R3
R2
R4
232Ω 32.4kΩ
1%
1%
100kΩ
1%
D
V_OUT
=
––––
4096
(5V)
OP191/OP291/OP491
The OP291 serves two functions. First, it is required to buffer
the high output impedance of the DAC’s V_REF pin, which is on
the order of 10 kΩ. The op amp provides a low impedance
output to drive any following circuitry. Secondly, the op amp
amplifies the output signal to provide a rail-to-rail output swing.
In this particular case, the gain is set to 4.1 to generate a 5.0 V
output when the DAC is at full scale. If other output voltage
ranges are needed, such as 0 to 4.095, the gain can easily be
adjusted by altering the value of the resistors.
A High Side Current Monitor
In the design of power supply control circuits, a great deal of
design effort is focused on ensuring a pass transistor’s long-term
reliability over a wide range of load current conditions. As a
result, monitoring and limiting device power dissipation is of
prime importance in these designs. The circuit illustrated in
Figure 63 is an example of a +5 V, single-supply high side
current monitor that can be incorporated into the design of a
voltage regulator with fold-back current limiting or a high
current power supply with crowbar protection. This design uses
an OP291’s rail-to-rail input voltage range to sense the voltage
drop across a 0.1 Ω current shunt. A p-channel MOSFET used
as the feedback element in the circuit converts the op amp’s
differential input voltage into a current. This current is then
applied to R2 to generate a voltage that is a linear representation
of the load current. The transfer equation for the current
monitor is given by:
Monitor
Output
=
R2
×


R_SENSE
R1


×
I_L
For the element values shown, the Monitor Output’s transfer
characteristic is 2.5 V/A.
+5V
R1
100Ω
R_SENSE
0.1Ω
S
M1
G
3N163
MONITOR
OUTPUT
D
R2
2.49kΩ
I_L
+5V
3
8
1/2
2 ^OP291
4
+5V
1
Figure 63. A High-Side Load Current Monitor
Figure 62. +5 V Only, 12-Bit DAC Swings Rail-to-Rail
REV. 0
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