AD8531/AD8532/AD8534
THEORY OF OPERATION
The AD8531/AD8532/AD8534 are all CMOS, high output
current drive, rail-to-rail input/output operational amplifiers.
Their high output current drive and stability with heavy capacitive
loads make the AD8531/AD8532/AD8534 excellent choices as
drive amplifiers for LCD panels.
SHORT-CIRCUIT PROTECTION
As a result of the design of the output stage for the maximum
load current capability, the AD8531/AD8532/AD8534 do not
have any internal short-circuit protection circuitry. Direct
connection of the output of the AD8531/AD8532/AD8534 to
the positive supply in single-supply applications destroys the
device. In applications where some protection is needed, but not
at the expense of reduced output voltage headroom, a low value
resistor in series with the output, as shown in Figure 37, can be
used. The resistor, connected within the feedback loop of the
amplifier, has very little effect on the performance of the amplifier
other than limiting the maximum available output voltage
swing. For single 5 V supply applications, resistors less than
20 Ω are not recommended.
Figure 36 illustrates a simplified equivalent circuit for the
AD8531/AD8532/AD8534. Like many rail-to-rail input amplifier
configurations, it comprises two differential pairs, one N-channel
(M1 to M2) and one P-channel (M3 to M4). These differential
pairs are biased by 50 μA 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
differential output currents. These differential output currents
are then combined in a compound folded-cascade second gain
stage (M5 to 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 using M11 to M14.
5V
R
20Ω
V
X
IN
V
AD8532
OUT
To achieve rail-to-rail output swings, the AD8531/AD8532/
AD8534 design employs a complementary, common source
output stage (M15 to M16). However, the output voltage swing
is directly dependent on the load current because the difference
between the output voltage and the supply is determined by
the AD8531/AD8532/AD8534’s output transistors on channel
resistance (see Figure 12 and Figure 13). 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
on the total load resistance at the output of the AD8531/
AD8532/AD8534.
Figure 37. Output Short-Circuit Protection
POWER DISSIPATION
Although the AD8531/AD8532/AD8534 are capable of
providing load currents to 250 mA, the usable output load
current drive capability is limited to the maximum power
dissipation allowed by the device package used. In any
application, the absolute maximum junction temperature
for the AD8531/AD8532/AD8534 is 150°C. The maximum
junction temperature should never be exceeded because the
device could suffer premature failure. Accurately measuring
power dissipation of an integrated circuit is not always a
straightforward exercise; therefore, Figure 38 is provided
as a design aid for either setting a safe output current drive
level or selecting a heat sink for the package options available
on the AD8531/AD8532/AD8534.
V+
50µA
M3
20µA
100µA
100µA
M11
M12
M5
1.5
V
M8
B2
T
MAX = 150°C
J
FREE AIR
NO HEAT SINK
M1
M4
M2
M15
OUT
M16
IN–
IN+
SOIC
= 158°C/W
M6
M7
θ
JA
1.0
V
M9
20µA
M10
SOT-23
= 230°C/W
B3
M14
θ
JA
MSOP
= 210°C/W
θ
JA
M13
50µA
SC70
JA
0.5
θ
= 376°C/W
V–
Figure 36. Simplified Equivalent Circuit
TSSOP
= 240°C/W
θ
JA
0
0
25
50
TEMPERATURE (°C)
75
85
100
Figure 38. Maximum Power Dissipation vs. Ambient Temperature
Rev. F | Page 11 of 20
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