OP495_15 Datasheet, PDF (13/16 Pages) Analog Devices – Dual/Quad Rail-to-Rail Operational Amplifiers

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.
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.
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.
SQUARE WAVE OSCILLATOR
The circuit in Figure 32 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.
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.
OP295/OP495
100kΩ
V+
58.7kΩ
100kΩ
3₊ 8
2– 4
1
1/2
OP295/
OP495
FREQ OUT
1
F_OSC = _RC < 350Hz @ V+ = 5V
+
C
R
Figure 32. Square Wave Oscillator Has Stable Frequency Regardless of
Supply Changes
90.9kΩ
2.2µF
V_IN ⁺
+
10kΩ
10kΩ
V+
–
+
1/4
OP295/
100kΩ OP495
SPEAKER
20kΩ
V+
–
–
+
+ 1/4
20kΩ
OP295/
OP495
1/4
OP295/
OP495
Figure 33. Single-Supply Differential Speaker Driver
HIGH ACCURACY, SINGLE-SUPPLY, LOW POWER
COMPARATOR
The OP295/OP495 make accurate open-loop comparators.
With a single 5 V supply, the offset error is less than 300 μV.
Figure 34 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.
1V
100
90
INPUT
(5mV OVERDRIVE
@ OP295 INPUT)
OUTPUT
10
0%
2V
5ms
Figure 34. Open-Loop Comparator Response Time with 5 mV Overdrive
Rev. G | Page 13 of 16