OP281_15 Datasheet, PDF (15/20 Pages) Analog Devices – Ultralow Power, Rail-to-Rail Output Operational Amplifiers

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
shunt resistor, RSENSE, creates a very small voltage drop. The
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
voltage drop across R1 equal to the voltage drop across RSENSE.
Therefore, the current through Q1 becomes directly
proportional to the current through RSENSE, and the output
voltage is given by the following equation:
V_OUT
= V_CC
−
⎜⎝⎛
R2
R1
× R_SENSE
× I _{L ⎟⎠⎞}
The voltage drop across R2 increases as IL increases; therefore,
VOUT decreases if a higher supply current is sensed. For the
element values shown, the VOUT transfer characteristic is
−2.5 V/A, decreasing from VCC.
V_CC
V_OUT
R2
2.49kΩ
Q1
V_CC
R1
100Ω
0.1Ω
R_SENSE
SINGLE
CHANNEL
OPx81
RETURN TO
GROUND
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.
OP281/OP481
R1
100kΩ
3V
2kΩ
V_IN = 2V p-p
A1
OP281-A
R2
100kΩ
3V
A2
OP281-B
FULL-WAVE
RECTIFIED
OUTPUT
HALF-WAVE
RECTIFIED
OUTPUT
Figure 43. Single-Supply Full-Wave and Half-Wave Rectifiers Using an OP281
100
90
SCALE
0.1V/DIV
10
SCALE
0%
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.
Rev. D | Page 15 of 20