OP450GRU AD [Analog Devices], OP450GRU Datasheet - Page 10

• Manufacturer Part Number: OP450GRU
• Description: CMOS Single-Supply Rail-to-Rail Input/Output Operational Amplifiers
• Manufacturer: AD [Analog Devices]

OP250/OP450
THEORY OF OPERATION
The OPx50 family of amplifiers are CMOS rail-to-rail input and
output single supply amplifiers designed for low cost and high
output current drive. These features make the OPx50 op amps
ideal for multimedia and telecom applications.
Figure 27 shows the simplified schematic for an OPx50 ampli-
fier. 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 differen-
tial pairs are combined in a compound folded-cascode stage,
which 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 configura-
tion. As with any rail-to-rail output amplifier, the gain of the
output stage, and thus the open loop gain of the amplifier, is de-
pendent on the load resistance. Also, the maximum output volt-
age 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
OPx50’s output transistors’ on-channel resistance. The output
dropout voltage is given in Figures 1 and 2.
Input Voltage Protection
Although not shown on the simplified schematic, there are ESD
protection diodes connected from each input to each power supply
rail. These diodes are normally reversed biased, but will turn on if
either input voltage exceeds either supply rail by more than 0.6 V.
Should this condition occur the input current should be limited to
less than 5 mA. This can be done by placing a resistor in series
with the input. The minimum resistor value should be:
–V
IN
BIAS
M1
M3
R
IN
M4
V
IN MAX
5
mA
M2
BIAS
BIAS
Figure 27. OPx50 Simplified Schematic
+V
IN
(1)
–10–
Output Phase Reversal
The OPx50 is immune to output voltage phase reversal with an
input voltage within the supply voltages of the device. However,
if either of the device’s inputs exceeds 0.6 V outside of the sup-
ply rails, the output could exhibit phase reversal. This is due to
the ESD protection diodes becoming forward biased, thus caus-
ing the polarity of the input terminals of the device to switch.
The technique recommended in the Input Overvoltage Protec-
tion section should be applied in applications where the possibil-
ity of input voltages exceeding the supply voltages exists.
Output Short Circuit Protection
To achieve high quality rail-to-rail performance, the outputs of
the OPx50 family are not short-circuit protected. Although
these amplifiers are designed to sink or source as much as
250 mA of output current, shorting the output directly to
ground could damage or destroy the device when excessive volt-
ages or currents are applied. If to protect the output stage, the
maximum output current should be limited to 250 mA.
By placing a resistor in series with the output of the amplifier as
shown in Figure 28, the output current can be limited. The
minimum value for R
For a +5 V single supply application, R
20 . Because R
fected. The trade-off in using R
voltage swing under heavy output current loads. R
increase the effective output impedance of the amplifier to
R
O
+ R
X
, where R
V
CC
X
O
is inside the feedback loop, V
V
EE
is the output impedance of the device.
X
can be found from Equation 2.
R
X
250
X
V
is a slight reduction in output
SY
mA
X
should be at least
OUT
X
is not af-
will also
M5
M6
REV. 0
V
OUT
(2)