AD8531ART Analog Devices, AD8531ART Datasheet - Page 8

• Manufacturer Part Number: AD8531ART
• Description: Low Cost, 250 ma Output Single-supply Amplifiers
• Manufacturer: Analog Devices

AD8531/AD8532/AD8534
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 A 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.
Response
Figure 29. Large Signal Transient
100
0%
90
10
500mV
R
V
A
T
500ns
S
A
V
L
= 2k
= +25 C
=
= 1
2.5V
Response
Figure 30. Large Signal Transient
100
0%
90
10
500mV
–8–
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
IN–
IN+
are not recommended.
V
A
R
T
M1
A
500ns
S
V
L
=
= 2k
= +25 C
= 1
Figure 33. Output Short-Circuit Protection
50 A
50 A
M3
1.35V
V
IN
M4 M2
V
V
AD8532
100 A
B2
B3
100
0%
90
10
+5V
Figure 31. No Phase Reversal
M5
M6
M7
1V
1V
V+
20
R
M8
X
V–
100 A
M9
M10
20 A
M12
M11
V
M13
OUT
M14
20 A
10 s
M16
M15
REV. B
OUT