OP196/OP296/OP496

REV. C

–14–

1/2

OP296

4

1

3

2

8

1000pF

44.2k

1%

30.9k

1%

AD589

43k

1.235V

MJE 350

100 F

5V TO 3.2V

Figure 8. 3 V Low Dropout Voltage Regulator

Figure 9 shows the regulator’s recovery characteristics when its

output underwent a 20 mA to 50 mA step current change.

10

0%

100

90

2V

50µs

10mV

50mA

30mA

OUTPUT

STEP

CURRENT

CONTROL

WAVEFORM

Figure 9. Output Step Load Current Recovery

Buffering a DAC Output

Multichannel TrimDACs® such as the AD8801/AD8803, are

widely used for digital nulling and similar applications. These

DACs have rail-to-rail output swings, with a nominal output

resistance of 5 k

Ω. If a lower output impedance is required, an

OP296 amplifier can be added. Two examples are shown in

Figure 10. One amplifier of an OP296 is used as a simple buffer

to reduce the output resistance of DAC A. The OP296 provides

rail-to-rail output drive while operating down to a 3 V supply

and requiring only 50

µA of supply current.

5V

OP296

SIMPLE BUFFER

0V TO 5V

+4.983V

+1.1mV

R1

100k

SUMMER CIRCUIT

WITH FINE TRIM

ADJUSTMENT

DIGITAL INTERFACING

OMITTED FOR CLARITY

AD8801/

AD8803

GND

Figure 10. Buffering a TrimDAC OutputTPC

The next two DACs, B and C, sum their outputs into the other

OP296 amplifier. In this circuit DAC C provides the coarse

output voltage setting and DAC B is used for fine adjustment.

The insertion of R1 in series with DAC B attenuates its contri-

bution to the voltage sum node at the DAC C output.

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 11

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 OP296’s rail-to-

rail input voltage range to sense the voltage drop across a 0.1

Ω

current shunt. A p-channel MOSFET is used as the feedback

element in the circuit to convert the op amp’s differential input

voltage into a current. This current is then applied to R2 to gen-

erate 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.

8

1

2

3

4

1/2

OP296

5V

5V

S

G

D

M1

3N163

MONITOR

OUTPUT

R2

2.49k

R1

100

0.1

5V

Figure 11. A High-Side Load Current Monitor

A Single-Supply RTD Amplifier

The circuit in Figure 12 uses three op amps on the OP496 to

produce a bridge driver for an RTD amplifier while operating

from a single 5 V supply. The circuit takes advantage of the

OP496’s wide output swing to generate a bridge excitation

voltage of 3.9 V. An AD589 provides a 1.235 V reference for

the bridge current. Op amp A1 drives the bridge to maintain

1.235 V across the parallel combination of the 6.19 k

Ω and

2.55 M

Ω resistors, which generates a 200 µA current source.

This current divides evenly and flows through both halves of

the bridge. Thus, 100

µA flows through the RTD to generate

an output voltage which is proportional to its resistance. For

improved accuracy, a 3-wire RTD is recommended to balance

the line resistance in both 100

Ω legs of the bridge.

TrimDAC is a registered trademark of Analog Devices Inc.