0

UNSTABLE

ROC = 40 dB/decade

STABLE

ROC ± 20 dB/decade

FREQUENCY (Hz)

13

LMV551-Q1

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SNOSD24 – FEBRUARY 2017

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Feature Description (continued)

7.3.5 Small Size

The small footprint of the DCK (SC-70) package saves space on printed circuit boards, and enable the design of

smaller and more compact electronic products. Long traces between the signal source and the op amp make the

signal path susceptible to noise. By using a physically smaller package, the amplifier can be placed closer to the

signal source, reducing noise pickup and enhancing signal integrity.

7.4 Device Functional Modes

7.4.1 Stability Of Op Amp Circuits

7.4.1.1 Stability and Capacitive Loading

As seen inFigure 26, the phase margin reduces significantly for CL greater than 100 pF. This is because the op

amp is designed to provide the maximum bandwidth possible for a low supply current. Stabilizing the amplifier for

higher capacitive loads would have required either a drastic increase in supply current, or a large internal

compensation capacitance, which would have reduced the bandwidth of the op amp. When the LMV551-Q1 is to

be used for driving higher capacitive loads, it must be externally compensated.

Figure 26. Gain vs Frequency for an Op Amp

An op amp, ideally, has a dominant pole close to DC, which causes its gain to decay at the rate of 20 dB/decade

with respect to frequency. If this rate of decay, also known as the rate of closure (ROC), remains the same until

the op amp’s unity gain bandwidth crosses zero, the op amp is stable. If, however, a large capacitance is added

to the output of the op amp, it combines with the output impedance of the op amp to create another pole in its

frequency response before its unity gain frequency (Figure 26). This increases the ROC to 40 dB/ decade and

causes instability.

In such a case a number of techniques can be used to restore stability to the circuit. The idea behind all these

schemes is to modify the frequency response such that it can be restored to an ROC of 20 dB/decade, which

ensures stability.

7.4.1.1.1

In the Loop Compensation

Figure 27 illustrates a compensation technique, known as in the loop compensation, that employs an RC

feedback circuit within the feedback loop to stabilize a non-inverting amplifier configuration. A small series

resistance, RS, is used to isolate the amplifier output from the load capacitance, CL, and a small capacitance, CF,

is inserted across the feedback resistor to bypass CL at higher frequencies.