OP777

–11–

REV. 0

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.

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

OP777

Figure 46. Follower Circuit

TIME – 10 s/DIV

1.0mV

Figure 47. Rail-to-Rail Operation

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

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 output voltage is given by:

VV

R

R

RI

OUT

SENSE

L

=−

×

×









5

2

1

decreases with higher supply current being sensed. For the element

5V

2.49k

OP777

5V

100

Q1

RETURN TO

GROUND

0.1

Figure 48. A Low-Side Load Current Monitor