AD8591 Analog Devices, AD8591 Datasheet - Page 11

• Manufacturer Part Number: AD8591
• Manufacturer: Analog Devices

Specifications of AD8591

• Vcc-vee: 2.5V to 6V
• Isy Per Amplifier: 1.25mA
• Packages: SOT
• -3db Bandwidth: 5MHz
• Slew Rate: 3.5V/µs
• Vos: 25mV
• Ib: 5pA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 30nV/rtHz

THEORY OF OPERATION
The AD859x amplifiers are CMOS, high output drive, rail-to-
rail input and output single-supply amplifiers designed for low
cost and high output current drive. The parts include a power
saving shutdown function that makes the AD8591/AD8592/
AD8594 op amps ideal for portable multimedia and
telecommunications applications.
Figure 33 shows the simplified schematic for the AD8591/AD8592/
AD8594 amplifiers. Two input differential pairs, consisting of
an n-channel pair (M1, M2) and a p-channel pair (M3, M4),
provide a rail-to-rail input common-mode range. The outputs of
the input differential pairs are combined in a compound folded-
cascode stage that drives the input to a second differential pair
gain stage. The outputs of the second gain stage provide the gate
voltage drive to the rail-to-rail output stage.
The rail-to-rail output stage consists of M15 and M16, which
are configured in a complementary common source configuration.
As with any rail-to-rail output amplifier, the gain of the output
stage, and thus the open-loop gain of the amplifier, is dependent
on the load resistance. In addition, the maximum output voltage
swing is directly proportional to the load current. The difference
between the maximum output voltage to the supply rails, known as
the dropout voltage, is determined by the on-channel resistance
of the AD8591/AD8592/AD8594 output transistors. The output
dropout voltage is given in Figure 5 and Figure 6.
SD
INPUT VOLTAGE PROTECTION
Although not shown in the simplified schematic, ESD protection
diodes are connected from each input to each power supply rail.
These diodes are normally reverse-biased, but turn on if either
input voltage exceeds either supply rail by more than 0.6 V. If this
condition occurs, limit the input current to less than ±5 mA.
This is done by placing a resistor in series with the input(s).
The minimum resistor value should be
*ALL CURRENT SOURCES GO TO 0µA IN SHUTDOWN MODE.
IN–
IN+
R ≥
INV
M1
IN
50µA
V
INV
M3
5
IN
mA
, MAX
*
*
50µA
M4
Figure 33. Simplified Schematic
V
M337
M2
M340
B3
V
100µA
B2
M5
M6
M7
*
V+
M8
V–
*
100µA
M9
M10
20µA
M12
M11
M13
*
M14
20µA
*
M30
M15
M16
M31
OUT
Rev. B | Page 11 of 16
(1)
OUTPUT PHASE REVERSAL
The AD8591/AD8592/AD8594 are immune to output voltage
phase reversal with an input voltage within the supply voltages
of the device. However, if either of the inputs of the device exceeds
0.6 V outside of the supply rails, the output could exhibit phase
reversal. This is due to the ESD protection diodes becoming
forward-biased, thus causing the polarity of the input terminals
of the device to switch.
The technique recommended in the Input Voltage Protection
section should be applied in applications where the possibility
of input voltages exceeding the supply voltages exists.
OUTPUT SHORT-CIRCUIT PROTECTION
To achieve high output current drive and rail-to-rail performance,
the outputs of the AD859x family do not have internal short-
circuit protection circuitry. Although these amplifiers are
designed to sink or source as much as 250 mA of output current,
shorting the output directly to the positive supply could damage or
destroy the device. To protect the output stage, limit the maximum
output current to ±250 mA.
By placing a resistor in series with the output of the amplifier,
as shown in Figure 34, the output current can be limited. The
minimum value for R
For a 5 V single-supply application, R
Because R
trade-off in using R
swing under heavy output current loads. R
effective output impedance of the amplifier to R
is the output impedance of the device.
POWER DISSIPATION
Although the AD859x amplifiers are able to provide load
currents of up to 250 mA, proper attention should be given to
not exceeding the maximum junction temperature for the device.
The junction temperature equation is
where:
T
P
θ
package.
T
JA
DISS
J
A
is the AD859x junction temperature.
is the ambient temperature of the circuit.
is the AD859x junction-to-ambient thermal resistance of the
T
is the AD859x power dissipation.
R ≥
J
X
= P
X
250
DISS
is inside the feedback loop, V
V
× θ
SY
Figure 34. Output Short-Circuit Protection
mA
V
JA
IN
+ T
X
is a slight reduction in output voltage
X
A
is
AD8592
AD8591/AD8592/AD8594
+5V
20Ω
R
X
X
should be at least 20 Ω.
X
OUT
V
also increases the
OUT
is not affected. The
O
+ R
X
, where R
(2)
(3)
O