OP777 Datasheet, PDF (11/12 Pages) Analog Devices – Precision Micropower Single Supply Operational Amplifier

OP777
Phase Reversal
Many amplifiers misbehave when one or both of the inputs are
forced beyond the input common-mode voltage range. Phase
reversal is typified by the transfer function of the amplifier, effectively
reversing its transfer polarity. In some cases this can cause lockup in
servo systems and may cause permanent damage or nonrecoverable
parameter shifts to the amplifier. Many amplifiers feature compensa-
tion circuitry to combat these effects, but some are only effective for
the inverting input. Additionally, many of these schemes only work
for a few hundred millivolts or so beyond the supply rails. OP777
has a protection circuit against phase reversal when one or both
inputs are forced beyond their input common voltage range. It
is not recommended that the parts be continuously driven more
than 3 V beyond the rails.
V_SY = ؎15V
V_IN
V_OUT
TIME – 400␮s/DIV
Figure 45. No Phase Reversal
Output Stage
The CMOS output stage has excellent (and fairly symmetric) output
drive and with light loads can actually swing to within 1 mV of both
supply rails. This is considerably better than similar amplifiers
featuring (so-called) rail-to-rail bipolar output stages. OP777 is
stable in the voltage follower configuration and responds to signals
as low as 1 mv above ground in single supply operation.
2.7V TO 30V
V_IN = 1mV
V_OUT = 1mV
OP777
Figure 46. Follower Circuit
Output Short Circuit
The output of the OP777 series amplifier is protected from damage
against accidental shorts to either supply voltage, provided that the
maximum die temperature is not exceeded on a long-term basis (see
Absolute Maximum Rating section). Current of up to 30 mA does
not cause any damage.
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 48 shows an example of 5 V, single supply
current monitor that can be incorporated into the design of a voltage
regulator with foldback current limiting or a high current power
supply with crowbar protection. The design capitalizes on the
OP777’s common-mode range that extends to ground. Current
is monitored in the power supply return 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 equiva-
lent 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
=
5V
−


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, decreas-
ing from V_EE.
5V
2.49k⍀
V_OUT
Q1
5V
100⍀
0.1⍀
R_SENSE
OP777
RETURN TO
GROUND
Figure 48. A Low-Side Load Current Monitor
1.0mV
TIME – 10␮s/DIV
Figure 47. Rail-to-Rail Operation
REV. 0
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