REV. C

OP777/OP727/OP747

–11–

Input Over Voltage Protection

When the input of an amplifier is more than a diode drop below

(V–) or the positive supply (V+), respectively, to the input pins

which can destroy the device. In the case of OP777/OP727/

OP747, differential voltages equal to the supply voltage will not

cause any problem (see Figure 3). OP777/OP727/OP747 has

built- in 500

Ω internal current limiting resistors, in series with the

inputs, to minimize the chances of damage. It is a good practice to

keep the current flowing into the inputs below 5 mA. In this con-

text it should also be noted that the high breakdown of the input

transistors removes the necessity for clamp diodes between the

inputs of the amplifier, a feature that is mandatory on many preci-

sion op amps. Unfortunately, such clamp diodes greatly interfere

with many application circuits such as precision rectifiers and

comparators. The OP777/OP727/OP747 series is free from such

limitations.

30V

V p-p = 32V

OP777/

OP727/

OP747

Figure 3a. Unity Gain Follower

TIME – 400 s/DIV

Figure 3b. Input Voltage Can Exceed the Supply Voltage

Without Damage

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/

OP727/OP747 has a protection circuit against phase reversal

when one or both inputs are forced beyond their input common-

mode voltage range. It is not recommended that the parts be

continuously driven more than 3 V beyond the rails.

TIME – 400 s/DIV

Figure 4. 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/

OP727/OP747 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

OP777/

OP727/

OP747

Figure 5. Follower Circuit

TIME – 10 s/DIV

1.0mV

Figure 6. Rail-to-Rail Operation

Output Short Circuit

The output of the OP777/OP727/OP747 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