SM73301 Datasheet, PDF (16/22 Pages) Texas Instruments – RRIO, High Output Current & Unlimited Cap Load Op Amp in SOT23-5

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FIGURE 2. Input Stage Current vs. Differential Input
Voltage
B) Output Stage
The output stage Figure 1 is comprised of complementary
NPN and PNP common-emitter stages to permit voltage
swing to within a V_CE(SAT) of either supply rail. Q9 supplies the
sourcing and Q10 supplies the sinking current load. Output
current limiting is achieved by limiting the V_CE of Q9 and Q10;
using this approach to current limiting, alleviates the draw
back to the conventional scheme which requires one V_BE re-
duction in output swing.
The frequency compensation circuit includes Miller capacitors
from collector to base of each output transistor (see Figure
1, C_comp9 and C_comp10). At light capacitive loads, the high fre-
quency 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 de-
crease the high frequency gain of the output transistors 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 compen-
sation, 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 stabil-
ity under all capacitive loads.
DRIVING CAPACITIVE LOADS
The SM73301 is specifically designed to drive unlimited ca-
pacitive loads without oscillations (See Settling Time and
Percent Overshoot vs. Cap Load plots in the typical perfor-
mance characteristics section). In addition, the output current
handling capability of the device allows for good slewing char-
acteristics even with large capacitive loads (see Slew Rate
vs. Cap Load plots). The combination of these features is ide-
al for applications such as TFT flat panel buffers, A/D con-
verter 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 Slew Rate vs.
Cap Load Plots (typical performance characteristics section),
two distinct regions can be identified. 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. Note that because of the
lower output sourcing current compared to the sinking one,
the Slew Rate limit under heavy capacitive loading is deter-
mined by the positive transitions. 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.
For the SM73301, the available output current increases with
the input overdrive. Referring to Figure 3 and Figure 4, Output
Short Circuit Current vs. Input Overdrive, it can be seen that
both sourcing and sinking short circuit current increase as in-
put overdrive increases. In a closed loop amplifier configura-
tion, 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 consump-
tion, while enabling the device to deliver large output current
when the need arises (such as during transients).
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FIGURE 3. Output Short Circuit Sourcing Current vs.
Input Overdrive
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