OP191_15 Datasheet, PDF (20/24 Pages) Analog Devices – Micropower Single-Supply Rail-to-Rail Input/Output Op Amps

OP191/OP291/OP491
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 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 67
is an example of a 2.5 V reference that operates from a single
3 V supply. The circuit takes advantage of the OP291 rail-to-rail
input and output voltage ranges to amplify an AD589 1.235 V
output to 2.5 V. The OP291 low TCVOS of 1 μV/°C helps
maintain an output voltage temperature coefficient of less than
200 ppm/°C. The circuit overall temperature coefficient is
dominated by the temperature coefficient of R2 and R3. Lower
temperature coefficient resistors are recommended. The entire
circuit draws less than 420 μA from a 3 V supply at 25°C.
3V
R1
17.4kΩ
AD589
3V
38
1/2
OP291
1
24
2.5V_REF
RESISTORS = 1%, 100ppm/°C
POTENTIOMETER = 10 TURN, 100ppm/°C
R3
100kΩ
R2
R1
100kΩ 5kΩ
Figure 67. A 2.5 V Reference that Operates on a Single 3 V Supply
5 V ONLY, 12-BIT DAC SWINGS RAIL-TO-RAIL
The OPx91 family is ideal for use with a CMOS DAC to
generate a digitally controlled voltage with a wide output range.
Figure 68 shows the DAC8043 used in conjunction with the
AD589 to generate a voltage output from 0 V to 1.23 V. The
DAC is operated in voltage switching mode, where the reference
is connected to the current output, IOUT, and the output voltage
is taken from the VREF 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
3
I_{OUT DAC8043}
R_FB ²
1
V_REF
GND CLK SR1 LD
4 7 65
DIGITAL
CONTROL
5V
3
8
1/2
OP291 ¹
2
4
V_OUT = ––D–– (5V)
4096
R3
232Ω
1%
R2
32.4kΩ
1%
R4
100kΩ
1%
Figure 68. 5 V Only, 12-Bit DAC Swings Rail-to-Rail
The OP291 serves two functions. First, it is required to buffer
the high output impedance of the DAC VREF pin, which is on the
order of 10 kΩ. The op amp provides a low impedance output
to drive any following circuitry. Second, 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 V to 4.095 V, 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 69 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 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
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 transfer
characteristic is 2.5 V/A.
5V
R1
100Ω
R_SENSE
0.1Ω
S
M1
G
3N163
MONITOR
D
OUTPUT
R2
2.49kΩ
I_L
5V
5V
3
8
1/2
OP291
1
2
4
Figure 69. A High-Side Load Current Monitor
Rev. E | Page 20 of 24