OP295/OP495
current budget with which the circuit must operate. T his 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.
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.
A 3 Volt Low-D r opout Linear Voltage Regulator
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. T his
“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.
Figure 10 shows a simple 3 V voltage regulator design. T he
regulator can deliver 50 mA load current while allowing a 0.2 V
dropout voltage. T he 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. T he additional base current is eas-
ily handled by the OP295/OP495 output.
T he amplifier servos the output to a constant voltage, which
feeds a portion of the signal to the error amplifier.
T he OP295’s rail-to-rail swing exacts higher gate drive to
the power MOSFET , providing a fuller enhancement to the
transistor. T he regulator exhibits 0.2 V dropout at 500 mA of
load current. At 1 amp output, the dropout voltage is typically
5.6 volts.
Higher output current, to 100 mA, is achievable at a higher
dropout voltage of 3.8 V.
IL < 50mA
MJE 350
RSENSE
0.1Ω
1/4W
IO (NORM) = 0.5A
IO (MAX) = 1A
VO
100µF
IRF9531
44.2k
S
D
VIN
5V TO 3.2V
+5V VO
1%
8
3
2
G
210k
1%
205k
1%
6V
1/2
OP295/
OP495
1
30.9k
1%
8
5
6
4
A2
7
1N4148
1000pF
1/2
OP295/
OP495
45.3k
1%
45.3k
1%
1.235V
AD589
100k
5%
0.01µF
43k
3
2
124k
1%
A1
124k
1
1%
1/2
OP295/
OP495
4
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.
2.500V
REF43
2
6
4
2V
100
Figure 12. Low Dropout, 500 m A Voltage Regulator with
Fold-Back Current Lim iting
50mA
90
STEP
CURRENT
CONTROL
Squar e Wave O scillator
WAVEFORM
20mA
T he circuit in Figure 13 is a square wave oscillator (note the
positive feedback). T he 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.
T he rail-to-rail swing ensures that the noninverting input sees
the full V+/2, rather than only a fraction of it.
OUTPUT
10
0%
20mV
1ms
Figure 11. Output Step Load Current Recovery
T he constant frequency comes from the fact that the 58.7 kΩ
feedback sets up Schmitt T rigger 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. T he
slew rate of the amplifier limits the oscillation frequency to a
maximum of about 800 Hz at a +5 V supply.
Low-D r opout, 500 m A Voltage Regulator with Fold-Back
Cur r ent Lim iting
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
Single Supply D iffer ential Speaker D r iver
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.
–10–
REV. B
ADI [ ADI ]
