AD8531A Analog Devices, AD8531A Datasheet - Page 8

• Manufacturer Part Number: AD8531A
• 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 ca-
pacitive loads makes the AD8531/AD8532/AD8534 an excellent
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 configura-
tions, it is comprised of a 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 differen-
tial output currents. These differential output currents are then
combined together in a compound folded-cascode 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 out-
put 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; and as a result the volt-
age 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.
100
0%
90
10
Figure 29. Large Signal Tran-
sient Response
500mV
R
V
A
T
500ns
A
S
L
V
=
= 1
= 2k
= +25 C
2.5V
Figure 30. Large Signal Transient
Response
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 ex-
pense of reduced output voltage headroom, a low value resistor
in series with the output, as shown in Figure 33, can be used.
The resistor, connected within the feedback loop of the ampli-
fier, will have very little effect on the performance of the ampli-
fier other than limiting the maximum available output voltage
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
B3
100
B2
AD8532
100 A
0%
90
10
Figure 31. No Phase Reversal
1V
+5V
1V
M5
M6
M7
V+
20
R
M8
X
V–
100 A
M9
M10
20 A
M12
M11
M13
V
OUT
M14
10 s
20 A
M15
M16
REV. 0
OUT