lm8272mmx National Semiconductor Corporation, lm8272mmx Datasheet - Page 11

• Manufacturer Part Number: lm8272mmx
• Description: Rrio, High Output Current & Unlimited Cap Load Op Amp In Miniature Package
• Manufacturer: National Semiconductor Corporation

Application Notes
BLOCK DIAGRAM AND OPEATIONAL DESCRIPTION
A) INPUT STAGE:
As can be seen from the simplified schematic in Figure 1, the
input stage consists of two distinct differential pairs (Q1-Q2
and Q3-Q4) in order to accommodate the full Rail-to-Rail
input common mode voltage range. The voltage drop across
R5, R6, R7 and R8 is kept to less than 200mV in order to
allow the input to exceed the supply rails. Q13 acts as a
switch to steer current away from Q3-Q4 and into Q1-Q2, as
the input increases beyond 1.4 of V
signal path from the bottom stage differential pair to the top
one and causes a subsequent increase in the supply current.
In transitioning from one stage to another, certain input stage
parameters (V
on which differential pair is “on” at the time. Input Bias
current, I
crosses the transition region. In addition, parameter such as
PSRR and CMRR which involve the input offset voltage will
also be effected by changes in V
pair transition region.
The input stage is protected with the combination of R9-R10
and D1, D2, D3 and D4 against differential input over-
voltages. This fault condition could otherwise harm the dif-
ferential pairs or cause offset voltage shift in case of pro-
longed over voltage. As shown in Figure 2, if this voltage
reaches approximately
and current flow is limited by the internal series resistors (R9
and R10). The Absolute Maximum Rating of
tial on V
variation, the point were the diodes turn on will change at the
rate of 5mV/˚C
FIGURE 1. Simplified Schematic Diagram
b
IN
, will change in value and polarity as the input
still needs to be observed. With temperature
OS
, I
b
, I
OS
±
, e
1.4V at 25˚C, the diodes turn on
n
, and i
CM
n
) are determined based
+
. This in turn shifts the
across the differential
±
10V differen-
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B) OUTPUT STAGE:
The output stage (see Figure 1) is comprised of complimen-
tary NPN and PNP common-emitter stages to permit voltage
swing to within a V
sourcing and Q10 supplies the sinking current load. Output
current limiting is achieved by limiting the V
using this approach to current limiting, alleviates the draw
back to the conventional scheme which requires one V
reduction in output swing.
The frequency compensation circuit includes Miller capaci-
tors from collector to base of each output transistor (see
Figure 1, C
high frequency gain of the output transistors is high, and the
Miller effect increases the effective value of the capacitors
thereby stabilizing the Op Amp. Large capacitive loads
greatly decrease the high frequency gain of the output tran-
sistors thus lowering the effective internal Miller capacitance
- the internal pole frequency increases at the same time a
low frequency pole is created at the Op Amp output due to
the large load capacitor. In this fashion, the internal dominant
pole compensation, which works by reducing the loop gain to
less than 0dB when the phase shift around the feedback
loop is more than 180˚, varies with the amount of capacitive
load and becomes less dominant when the load capacitor
has increased enough. Hence the Op Amp is very stable
even at high values of load capacitance resulting in the
uncharacteristic feature of stability under all capacitive loads.
C) OUTPUT VOLTAGE SWING CLOSE TO V
The LM8272’s output stage design allows voltage swings to
within millivolts of either supply rail for maximum flexibility
and improved useful range. Because of this design architec-
ture, as can be seen from Figure 1 diagram, with Output
approaching either supply rail, either Q9 or Q10 Collector-
Base junction reverse bias will decrease. With output less
than a V
tor operates near saturation. In this mode of operation, the
transistor will exhibit higher junction capacitance and lower f
which will reduce Phase Margin. With the Noise Gain (NG =
1 + Rf/Rg, Rf & Rg are external gain setting resistors) of 2 or
higher, there is sufficient Phase Margin that this reduction (in
Phase Margin) is of no consequence. However, with lower
FIGURE 2. Input Stage Current vs. Differential Input
be
from either rail, the corresponding output transis-
comp9
and C
ce(sat)
comp10
of either supply rail. Q9 supplies the
Voltage
). At light capacitive loads, the
ce
of Q9 and Q10;
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101308A4
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