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

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).

Ω resis-

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-

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 600

Ω load can

accommodate an output swing to within 100mV of V

250mV of V

unclipped swing.

10130890

FIGURE 3. Steady State Output Sourcing

Characteristics with Load Lines

10130891

FIGURE 4. Steady State Output Sinking Characteristics

with Load Lines

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