OP281/OP481

Rev. C | Page 15 of 20

LOW-SIDE CURRENT MONITOR

In the design of power-supply control circuits, a great deal of

design effort is focused on ensuring the long-term reliability of

a pass transistor over a wide range of load current conditions.

As a result, monitoring and limiting device power dissipation is

of primary importance in these designs. Figure 42 shows an

example of a 5 V, single-supply 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. The design capitalizes on the OPx81’s

common-mode range extending to ground. Current is

monitored in the power-supply return path, where a 0.1 Ω

voltage at the inverting terminal becomes equal to the voltage at

the noninverting terminal through the feedback of Q1, which is

a 2N2222 or an equivalent NPN transistor. This makes the

Therefore, the current through Q1 becomes directly

voltage is given by the following equation:

⎟

⎠

⎞

⎜

⎝

⎛

×

×

−

=

L

SENSE

EE

OUT

I

R

R

R

V

V

1

2

5V

RETURN TO

GROUND

5V

R2

2.49kΩ

R1

100Ω

0.1Ω

Q1

SINGLE

CHANNEL

OPx81

Figure 42. Low-Side Load Current Monitor

LOW VOLTAGE HALF-WAVE AND FULL-WAVE

RECTIFIERS

Because of its quick overdrive recovery time, an OP281 can be

configured as a full-wave rectifier for low frequency (<500 Hz)

applications. Figure 43 shows the schematic.

3V

OP281-A

2kΩ

3V

OP281-B

A2

R1

100kΩ

R2

100kΩ

FULL-WAVE

RECTIFIED

OUTPUT

HALF-WAVE

RECTIFIED

OUTPUT

A1

Figure 43. Single-Supply Full-Wave and Half-Wave Rectifiers Using an OP281

10

0%

100

90

SCALE

0.1V/DIV

SCALE

0.1ms/DIV

Figure 44. Full-Wave Rectified Signal

Amplifier A1 is used as a voltage follower that tracks the input

voltage only when it is greater than 0 V. This provides a half-

wave rectification of the input signal to the noninverting

terminal of Amplifier A2. When A1’s output is following the

input, the inverting terminal of A2 also follows the input from

the virtual ground between the inverting and noninverting

terminals of A2. With no potential difference across R1, no

current flows through either R1 or R2; therefore, the output of

A2 also follows the input. When the input voltage goes below

0 V, the noninverting terminal of A2 becomes 0 V. This makes

A2 work as an inverting amplifier with a gain of 1 and provides

a full-wave rectified version of the input signal. A 2 kΩ resistor

in series with A1’s noninverting input protects the device when

the input signal becomes less than ground.

BATTERY-POWERED TELEPHONE HEADSET

AMPLIFIER

Figure 45 shows how the OP281 can be used as a two-way

amplifier in a telephone headset. One side of the OP281 can be

used as an amplifier for the microphone, and the other side can

be used to drive the speaker. A typical telephone headset uses a

600 Ω speaker and an electret microphone that requires a

supply voltage and a biasing resistor.