LMC6024 Datasheet, PDF (9/22 Pages) National Semiconductor (TI) – Low Power CMOS Quad Operational Amplifier

LMC6024
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APPLICATION HINTS
SNOS621D – AUGUST 2000 – REVISED MARCH 2013
AMPLIFIER TOPOLOGY
The topology chosen for the LMC6024 is unconventional (compared to general-purpose op amps) in that the
traditional unity-gain buffer output stage is not used; instead, the output is taken directly from the output of the
integrator, to allow rail-to-rail output swing. Since the buffer traditionally delivers the power to the load, while
maintaining high op amp gain and stability, and must withstand shorts to either rail, these tasks now fall to the
integrator.
As a result of these demands, the integrator is a compound affair with an embedded gain stage that is doubly fed
forward (via C_f and C_ff) by a dedicated unity-gain compensation driver. In addition, the output portion of the
integrator is a push-pull configuration for delivering heavy loads. While sinking current the whole amplifier path
consists of three gain stages with one stage fed forward, whereas while sourcing the path contains four gain
stages with two fed forward.
Figure 25. LMC6024 Circuit Topology (Each Amplifier)
The large signal voltage gain while sourcing is comparable to traditional bipolar op amps, for load resistance of at
least 5 kΩ. The gain while sinking is higher than most CMOS op amps, due to the additional gain stage;
however, when driving load resistance of 5 kΩ or less, the gain will be reduced as indicated in the Electrical
Characterisitics. The op amp can drive load resistance as low as 500Ω without instability.
COMPENSATING INPUT CAPACITANCE
Refer to the LMC660 or LMC662 datasheets to determine whether or not a feedback capacitor will be necessary
for compensation and what the value of that capacitor would be.
CAPACITIVE LOAD TOLERANCE
Like many other op amps, the LMC6024 may oscillate when its applied load appears capacitive. The threshold of
oscillation varies both with load and circuit gain. The configuration most sensitive to oscillation is a unity-gain
follower. See the Typical Performance Characteristics.
The load capacitance interacts with the op amp's output resistance to create an additional pole. If this pole
frequency is sufficiently low, it will degrade the op amp's phase margin so that the amplifier is no longer stable at
low gains. The addition of a small resistor (50Ω to 100Ω) in series with the op amp's output, and a capacitor (5
pF to 10 pF) from inverting input to output pins, returns the phase margin to a safe value without interfering with
lower-frequency circuit operation. Thus, larger values of capacitance can be tolerated without oscillation. Note
that in all cases, the output will ring heavily when the load capcitance is near the threshold for oscillation.
Figure 26. Rx, Cx Improve Capacitive Load Tolerance
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