LMC6035 Datasheet, PDF (15/20 Pages) National Semiconductor (TI) – Low Power 2.7V Single Supply CMOS Operational Amplifiers

1.0 Application Notes (Continued)
(a) Inverting Amplifier
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(c) Follower
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(b) Non-Inverting Amplifier
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(d) Howland Current Pump
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FIGURE 9. Guard Ring Connections
1.3.1 CAPACITIVE LOAD TOLERANCE
Like many other op amps, the LMC6035/6 may oscillate
when its applied load appears capacitive. The threshold of
oscillation varies both with load and circuit gain. The con-
figuration 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. As
shown in Figure 10, the addition of a small resistor
(50Ω–100Ω) in series with the op amp’s output, and a ca-
pacitor (5pF–10pF) from inverting input to output pins, re-
turns 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 capaci-
tance is near the threshold for oscillation.
1.4 Micro SMD Considerations
Contrary to what might be guessed, the micro SMD package
does not follow the trend of smaller packages having higher
thermal resistance. LMC6035 in micro SMD has thermal
resistance of 220˚C/W compared to 230˚C/W in MSOP. Even
when driving a 600Ω load and operating from ±7.5V sup-
plies, the maximum temperature rise will be under 4.5˚C. For
application information specific to micro SMD, see Applica-
tion note AN-1112.
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FIGURE 10. Rx, Cx Improve Capacitive Load Tolerance
Capacitive load driving capability is enhanced by using a pull
up resistor to V⁺ (Figure 11). Typically a pull up resistor
conducting 500µA or more will significantly improve capaci-
tive load responses. The value of the pull up resistor must be
determined based on the current sinking capability of the
amplifier with respect to the desired output swing. Open loop
gain of the amplifier can also be affected by the pull up
resistor (see Electrical Characteristics).
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