lm8272mmx National Semiconductor Corporation, lm8272mmx Datasheet - Page 12

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

www.national.com
Application Notes
Noise Gain (
supply rail, if the output loading is light, the Phase Margin
reduction could result in unwanted oscillations.
In the case of the LM8272, due to inherent architectural
specifics, the oscillation occurs only with respect to Q10
when output swings to within 150mV of V
collector current is larger than its idle value of a few micro-
amps, the Phase Margin loss becomes insignificant. In this
case, 300µA is the required Q10 collector current to remedy
this situation. Therefore, when all the aforementioned critical
conditions are present at the same time (NG
150mV from supply rails, & output load is light) it is possible
to ensure stability by adding a load resistor to the output to
provide the necessary Q10 minimum Collector Current
(300µA).
For 12V (or
tor from the output to V
current and ensure stability. This is equivalent to about 15%
increase in total quiescent power dissipation.
DRIVING CAPACTIVE LOADS:
The LM8272 is specifically designed to drive unlimited ca-
pacitive loads without oscillations (see Settling Time and
Overshoot vs. Cap Load plots in the typical performance
characteristics section). In addition, the output current han-
dling capability of the device allows for good slewing char-
acteristics even with large capacitive loads (Settling Time
and Slew Rate vs. Cap Load plot). The combination of these
features is ideal for applications such as TFT flat panel
buffers, A/D converter input amplifiers, etc.
However, as in most Op Amps, addition of a series isolation
resistor between the Op Amp and the capacitive load im-
proves the settling and overshoot performance.
Output current drive is an important parameter when driving
capacitive loads. This parameter will determine how fast the
output voltage can change. Referring to the Settling Time
and Slew Rate vs. Cap Load plots (typical performance
characteristics section), two distinct regions can be identi-
fied. Below about 10,000pF, the output Slew Rate is solely
determined by the Op Amp’s compensation capacitor value
and available current into that capacitor. Beyond 10nF, the
Slew Rate is determined by the Op Amp’s available output
current. An estimate of positive and negative slew rates for
loads larger than 100nF can be made by dividing the short
circuit current value by the capacitor.
ESTIMATING THE OUTPUT VOLTAGE SWING
It is important to keep in mind that the steady state output
current will be less than the current available when there is
an input overdrive present. For steady state conditions, Fig-
ure 3 and Figure 4 plots can be used to predict the output
swing. These plots also show several load lines correspond-
±
<
6V) operation, for example, add a 39kΩ resis-
2) and with less than 150mV voltage to the
+
to cause 300µA output sinking
(Continued)
−
. However, if Q10
<
2, V
OUT
<
12
ing to loads tied between the output and ground. In each
case, the intersection of the device plot at the appropriate
temperature with the load line would be the typical output
swing possible for that load. For example, a 600Ω load can
accommodate an output swing to within 100mV of V
250mV of V
unclipped swing.
FIGURE 4. Steady State Output Sinking Characteristics
FIGURE 3. Steady State Output Sourcing
+
Characteristics with Load Lines
(V
S
=
±
5V) corresponding to a typical 9.65V
with Load Lines
10130891
10130890
−
and to
PP