OP191GS datasheet(15/20 Pages) AD

15 / 20 page

OP191/OP291/OP491

REV. 0

–15–

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 TCVOS

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.

RESISTORS = 1%, 100ppm/

°C

POTENTIOMETER = 10 TURN, 100ppm/

°C

100k

Ω

1/2

OP291

100k

Ω

+3V

Ω

+2.5V

REF

17.4k

Ω

AD589

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, 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.

1/2

OP291

+5V

17.8k

Ω

AD589

232

Ω

32.4k

Ω

100k

Ω

OUT = –––– (5V)

4096

GND CLK SR1

DIGITAL

CONTROL

REF

OUT

1.23V

+5V

DAC-8043

Figure 62. +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’s VREF pin, which is on

the order of 10 k

Ω. 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

SENSE









× I

For the element values shown, the Monitor Output’s transfer

characteristic is 2.5 V/A.

1/2

OP291

+5V

SENSE

0.1

Ω

+5V

3N163

2.49k

Ω

MONITOR

OUTPUT

100

Ω

Figure 63. A High-Side Load Current Monitor

OP191GS Datasheet(PDF) 15 Page - Analog Devices

OP191GS Datasheet(HTML) 15 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL