lm7322mmx National Semiconductor Corporation, lm7322mmx Datasheet - Page 17

• Manufacturer Part Number: lm7322mmx
• Description: Lm7321 Single/ Lm7322 Dual Rail-to-rail Input/output, ?15v, High Output Current And Unlimited Capacitive Load Operational Amplifier
• Manufacturer: National Semiconductor Corporation

Application Information
DRIVING CAPACITIVE LOADS
The LM7321/LM7322 are specifically designed to drive un-
limited capacitive loads without oscillations as shown in
Figure 1.
In addition, the output current handling capability of the device
allows for good slewing characteristics even with large ca-
pacitive loads as shown in Figure 2 and Figure 3.
FIGURE 1. ±5% Settling Time vs. Capacitive Load
FIGURE 2. +SR vs. Capacitive Load
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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 improves
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 Slew Rate vs.
Capacitive Load Plots (typical performance characteristics
section), two distinct regions can be identified. Below about
10,000 pF, the output Slew Rate is solely determined by the
op amp’s compensation capacitor value and available current
into that capacitor. Beyond 10 nF, the Slew Rate is deter-
mined by the op amp’s available output current. Note that
because of the lower output sourcing current compared to the
sinking one, the Slew Rate limit under heavy capacitive load-
ing is determined by the positive transitions. An estimate of
positive and negative slew rates for loads larger than 100 nF
can be made by dividing the short circuit current value by the
capacitor.
For the LM7321/LM7322, the available output current in-
creases with the input overdrive. Referring to Figure 4 and
Figure 5, Output Short Circuit Current vs. Input Overdrive, it
can be seen that both sourcing and sinking short circuit cur-
rent increase as input overdrive increases. In a closed loop
amplifier configuration, during transient conditions while the
fed back output has not quite caught up with the input, there
will be an overdrive imposed on the input allowing more output
current than would normally be available under steady state
condition. Because of this feature, the op amp’s output stage
quiescent current can be kept to a minimum, thereby reducing
power consumption, while enabling the device to deliver large
output current when the need arises (such as during tran-
sients).
FIGURE 3. −SR vs. Capacitive Load
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