AD8531/AD8532/AD8534

REV. D

–8–

100

90

10

0%

500mV

500ns

Figure 29. Large Signal Transient

Response

APPLICATIONS

THEORY OF OPERATION

The AD8531/AD8532/AD8534 is an all-CMOS, high output

current drive, rail-to-rail input/output operational amplifier.

This is the latest entry in Analog Devices’ expanding family of

single-supply devices for the multimedia and telecom market-

places. Its high output current drive and stability with heavy

capacitive loads makes the AD8531/AD8532/AD8534 an excel-

lent choice as a drive amplifier for LCD panels.

Figure 32 illustrates a simplified equivalent circuit for the AD8531/

AD8532/AD8534. Like many rail-to-rail input amplifier configu-

rations, it is comprised of two differential pairs, one n-channel

(M1–M2) and one p-channel (M3–M4). These differential pairs

are biased by 50

mA current sources, each with a compliance

limit of approximately 0.5 V from either supply voltage rail. The

differential input voltage is then converted into a pair of differ-

ential output currents. These differential output currents are

then combined in a compound folded-cascade second gain

stage (M5–M9). The outputs of the second gain stage at M8

and M9 provide the gate voltage drive to the rail-to-rail output

stage. Additional signal current recombination for the output

stage is achieved through the use of transistors M11–M14.

In order to achieve rail-to-rail output swings, the AD8531/

AD8532/AD8534 design employs a complementary common-

source output stage (M15–M16). However, the output voltage

swing is directly dependent on the load current, as the difference

between the output voltage and the supply is determined by the

AD8531/AD8532/AD8534’s output transistors on-channel

resistance (see Figures 8 and 9). The output stage also exhibits

voltage gain by virtue of the use of common-source amplifiers;

as a result, the voltage gain of the output stage (thus, the open-

loop gain of the device) exhibits a strong dependence to the total

load resistance at the output of the AD8531/AD8532/AD8534.

50 A

100 A

100 A

20 A

M5

M8

M12

M15

M16

M11

OUT

M3

M4

M2

M1

IN–

IN+

M6

M7

M10

20 A

M13

50 A

V+

V–

M9

M14

Figure 32. AD8531/AD8532/AD8534 Simplified Equivalent

Circuit

Short-Circuit Protection

As a result of the design of the output stage for maximum load

current capability, the AD8531/AD8532/AD8534 does not have

any internal short-circuit protection circuitry. Direct connection of

the AD8531/AD8532/AD8534’s output to the positive supply

in single-supply applications will destroy the device. In those

applications where some protection is needed, but not at the

expense of reduced output voltage headroom, a low value resis-

tor in series with the output, as shown in Figure 33, can be

used. The resistor, connected within the feedback loop of the

amplifier, will have very little effect on the performance of the

amplifier other than limiting the maximum available output volt-

age swing. For single 5 V supply applications, resistors less than

20

W are not recommended.

5V

20

AD8532

Figure 33. Output Short-Circuit Protection

500ns

500mV

100

90

10

0%

Figure 30. Large Signal Transient

Response

1V

10 s

1V

0%

10

90

100

Figure 31. No Phase Reversal