REV. B

–10–

OP295/OP495

current budget with which the circuit must operate. This circuit

consumes only 1.4 mA maximum quiescent current, making 2.6

mA of current available to power additional signal conditioning

circuitry or to power a bridge circuit.

A 3 Volt Low-Dropout Linear Voltage Regulator

Figure 10 shows a simple 3 V voltage regulator design. The

regulator can deliver 50 mA load current while allowing a 0.2 V

dropout voltage. The OP295/OP495’s rail-to-rail output swing

handily drives the MJE350 pass transistor without requiring spe-

cial drive circuitry. At no load, its output can swing less than the

pass transistor’s base-emitter voltage, turning the device nearly

off. At full load, and at low emitter-collector voltages, the tran-

sistor beta tends to decrease. The additional base current is eas-

ily handled by the OP295/OP495 output.

The amplifier servos the output to a constant voltage, which

feeds a portion of the signal to the error amplifier.

Higher output current, to 100 mA, is achievable at a higher

dropout voltage of 3.8 V.

1000pF

43k

44.2k

1%

30.9k

1%

AD589

1.235V

V

O

100

µF

I

MJE 350

V

IN

5V TO 3.2V

1/2

OP295/

OP495

8

1

4

2

3

Figure 10. 3 V Low Dropout Voltage Regulator

Figure 11 shows the regulator’s recovery characteristic when its

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

10

100

0%

90

1ms

20mV

2V

50mA

20mA

OUTPUT

STEP

CURRENT

CONTROL

WAVEFORM

Figure 11. Output Step Load Current Recovery

Low-Dropout, 500 mA Voltage Regulator with Fold-Back

Current Limiting

Adding a second amplifier in the regulation loop as shown in

Figure 12 provides an output current monitor as well as fold-

back current limiting protection.

Amplifier A1 provides error amplification for the normal voltage

regulation loop. As long as the output current is less than 1 am-

pere, amplifier A2’s output swings to ground, reverse biasing the

diode and effectively taking itself out of the circuit. However, as

the output current exceeds 1 amp, the voltage that develops

across the 0.1

Ω sense resistor forces the amplifier A2’s output

to go high, forward-biasing the diode, which in turn closes the

current limit loop. At this point A2’s lower output resistance

dominates the drive to the power MOSFET transistor, thereby

effectively removing the A1 voltage regulation loop from the

circuit.

If the output current greater than 1 amp persists, the current

limit loop forces a reduction of current to the load, which causes

a corresponding drop in output voltage. As the output voltage

drops, the current limit threshold also drops fractionally, result-

ing in a decreasing output current as the output voltage de-

creases, to the limit of less than 0.2 A at 1 V output. This

“fold-back” effect reduces the power dissipation considerably

during a short circuit condition, thus making the power supply

far more forgiving in terms of the thermal design requirements.

Small heat sinking on the power MOSFET can be tolerated.

The OP295’s rail-to-rail swing exacts higher gate drive to

the power MOSFET, providing a fuller enhancement to the

transistor. The regulator exhibits 0.2 V dropout at 500 mA of

load current. At 1 amp output, the dropout voltage is typically

5.6 volts.

124k

1%

124k

1%

REF43

4

6

2

0.01

µF

100k

5%

210k

1%

205k

1%

45.3k

1%

45.3k

1%

R

SENSE

0.1

Ω

1/4W

+5V

V

O

I

I

S

D

G

IRF9531

1N4148

2.500V

A1

A2

1

3

2

8

7

4

5

6

6V

1/2

OP295/

OP495

1/2

OP295/

OP495

Figure 12. Low Dropout, 500 mA Voltage Regulator with

Fold-Back Current Limiting

Square Wave Oscillator

The circuit in Figure 13 is a square wave oscillator (note the

positive feedback). The rail-to-rail swing of the OP295/OP495

helps maintain a constant oscillation frequency even if the sup-

ply voltage varies considerably. Consider a battery powered sys-

tem where the voltages are not regulated and drop over time.

The rail-to-rail swing ensures that the noninverting input sees

the full V+/2, rather than only a fraction of it.

The constant frequency comes from the fact that the 58.7 k

Ω

feedback sets up Schmitt Trigger threshold levels that are di-

rectly proportional to the supply voltage, as are the RC charge

voltage levels. As a result, the RC charge time, and therefore the

frequency, remains constant independent of supply voltage. The

slew rate of the amplifier limits the oscillation frequency to a

maximum of about 800 Hz at a +5 V supply.

Single Supply Differential Speaker Driver

Connected as a differential speaker driver, the OP295/OP495

can deliver a minimum of 10 mA to the load. With a 600

Ω

load, the OP295/OP495 can swing close to 5 volts peak-to-peak

across the load.