LMC6035IBP datasheet(15/20 Pages) NSC

15 / 20 page

1.0 Application Notes (Continued)

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

(50

Ω–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 600

Ω 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.

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).

01283008

(a) Inverting Amplifier

01283009

(b) Non-Inverting Amplifier

01283010

01283011

(d) Howland Current Pump

FIGURE 9. Guard Ring Connections

01283005

FIGURE 10. Rx, Cx Improve Capacitive Load Tolerance

LMC6035IBP Datasheet(PDF) 15 Page - National Semiconductor (TI)

LMC6035IBP Datasheet(HTML) 15 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL