OP295/OP495

Rev. E | Page 13 of 16

V+

100kΩ

100kΩ

58.7kΩ

R

FREQ OUT

4

1

8

3

2

C

1

RC

< 350Hz @ V+ = 5V

1/2

OP295/

OP495

+

–

+

Amplifier A1 provides error amplification for the normal

voltage regulation loop. As long as the output current is less

than 1 A, the output of Amplifier A2 swings to ground, reverse-

biasing the diode and effectively taking itself out of the circuit.

However, as the output current exceeds 1 A, the voltage that

develops across the 0.1 Ω sense resistor forces the output of

Amplifier A2 to go high, forward-biasing the diode, which in

turn closes the current-limit loop. At this point, the A2’s lower

output resistance dominates the drive to the power MOSFET

transistor, thereby effectively removing the A1 voltage regula-

tion loop from the circuit.

Figure 31. Square Wave Oscillator Has Stable Frequency Regardless of

Supply Changes

If the output current greater than 1 A 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,

resulting in a decreasing output current as the output voltage

decreases, 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.

10kΩ

90.9kΩ

V+

100kΩ

10kΩ

20kΩ

20kΩ

V+

SPEAKER

+

+

1/4

OP295/

OP495

1/4

OP295/

OP495

1/4

OP295/

OP495

2.2µF

–

+

–

+

–

+

The rail-to-rail swing of the OP295 exacts higher gate drive to

the power MOSFET, providing a fuller enhancement to the tran-

sistor. The regulator exhibits 0.2 V dropout at 500 mA of load

current. At 1 A output, the dropout voltage is typically 5.6 V.

Figure 32. Single-Supply Differential Speaker Driver

HIGH ACCURACY, SINGLE-SUPPLY, LOW POWER

COMPARATOR

SQUARE WAVE OSCILLATOR

The OP295/OP495 make accurate open-loop comparators.

With a single 5 V supply, the offset error is less than 300 μV.

Figure 33 shows the response time of the OP295/OP495 when

operating open-loop with 4 mV overdrive. They exhibit a 4 ms

response time at the rising edge and a 1.5 ms response time at

the falling edge.

The circuit in Figure 31 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 supply

voltage varies considerably. Consider a battery-powered system

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.

10

100

0%

90

5ms

2V

1V

OUTPUT

INPUT

(5mV OVERDRIVE

@ OP295 INPUT)

The constant frequency comes from the fact that the 58.7 kΩ

feedback sets up Schmitt trigger threshold levels that are directly

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

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

remain constant independent of supply voltage. The slew rate of

the amplifier limits 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 V p-p across the load.

Figure 33. Open-Loop Comparator Response Time with 5 mV Overdrive