OP181 Datasheet, PDF (12/16 Pages) Analog Devices – Ultralow Power, Rail-to-Rail Output Operational Amplifiers

OP181/OP281/OP481
+3V
R1
V_H
R2
V_IN
2kΩ
+3V
+3V
D1
A1
OP281-A
+3V
+3V
5.1kΩ
V_OUT
10kΩ
Q1
5.1kΩ
R3 _{VL A2}
D2
OP281-B
R4
Figure 39. Using the OP281 as a Window Comparator
The threshold limits for the window are set by V_H and V_L,
provided that V_H > V_L. The output of A1 will stay at the
negative rail, in this case ground, as long as the input voltage is
less than V_H. Similarly, the output of A2 will stay at ground as
long the input voltage is higher than V_L. As long as V_IN remains
between V_L and V_H, the outputs of both op amps will be 0 V.
With no current flowing in either D1 or D2, the base of Q1 will
stay at ground, putting the transistor in cutoff and forcing V_OUT
to the positive supply rail. If the input voltage rises above V_H,
the output of A2 stays at ground, but the output of A1 will go to
the positive rail, and D1 will conduct current. This creates a
base voltage that will turn on Q1 and drive V_OUT low. The same
condition occurs if V_IN falls below V_L with A2’s output going
high, and D2 conducting current. Therefore, V_OUT will be high
if the input voltage is between V_L and V_H, and V_OUT will be low
if the input voltage moves outside of that range.
The R1 and R2 voltage divider sets the upper window voltage,
and the R3 and R4 voltage divider sets the lower voltage for the
window. For the window comparator to function properly, V_H
must be a greater voltage than V_L.
V_H
=
R2
R1 + R2
V_L
=
R4
R3 + R4
The 2 kΩ resistor connects the input voltage to the input termi-
nals to the op amps. This protects the OP281 from possible
excess current flowing into the input stages of the devices. D1
and D2 are small-signal switching diodes (1N4446 or equiva-
lent), and Q1 is a 2N2222 or equivalent NPN transistor.
A Low-Side Current Monitor
In the design of power supply control circuits, a great deal of
design effort is focused on ensuring a pass transistor’s long-term
reliability over a wide range of load current conditions. As a
result, monitoring and limiting device power dissipation is of
prime importance in these designs. Figure 40 shows an example
of a +5 V, single-supply current monitor that can be incorpo-
rated 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 OP181’s common-
mode range that extends to ground. Current is monitored in the
power supply return path where a 0.1 Ω shunt resistor, R_SENSE,
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
equivalent NPN transistor. This makes the voltage drop across
R1 equal to the voltage drop across R_SENSE. Therefore, the
current through Q1 becomes directly proportional to the current
through R_SENSE, and the output voltage is given by:
V_OUT
= V_EE
−


R2
R1
×
R_SENSE
×
I
L


The voltage drop across R2 increases with I_L increasing, so
V_OUT decreases with higher supply current being sensed. For
the element values shown, the V_OUT transfer characteristic is
–2.5 V/A, decreasing from V_EE.
+5V
V_OUT
R2
2.49kΩ
Q1
+5V
R1
100Ω
0.1Ω
R_SENSE
OP181
RETURN TO
GROUND
Figure 40. A 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 41 shows the schematic.
R1 = 100kΩ
R2 = 100kΩ
2kΩ
V_IN = 2V p-p
+3V
A1
OP281-A
+3V
A2
OP281-B
FULL-WAVE
RECTIFIED
OUTPUT
HALF-WAVE
RECTIFIED
OUTPUT
Figure 41. Single Supply Full- and Half-Wave Rectifiers
Using an OP281
100
90
SCALE 0.1V/DIV
10
0%
SCALE 0.1ms/DIV
Figure 42. Full-Wave Rectified Signal
–12–
REV. 0