Datasheet

LMV821-N, LMV822-N, LMV822-N-Q1
LMV824-N, LMV824-N-Q1
SNOS032G –AUGUST 1999–REVISED NOVEMBER 2013 www.ti.com
APPLICATION NOTE
This application note is divided into two sections: design considerations and Application Circuits.
DESIGN CONSIDERATIONS
This section covers the following design considerations:
1. Frequency and Phase Response Considerations
2. Unity-Gain Pulse Response Considerations
3. Input Bias Current Considerations
FREQUENCY AND PHASE RESPONSE CONSIDERATIONS
The relationship between open-loop frequency response and open-loop phase response determines the closed-
loop stability performance (negative feedback). The open-loop phase response causes the feedback signal to
shift towards becoming positive feedback, thus becoming unstable. The further the output phase angle is from
the input phase angle, the more stable the negative feedback will operate. Phase Margin (φ
m
) specifies this
output-to-input phase relationship at the unity-gain crossover point. Zero degrees of phase-margin means that
the input and output are completely in phase with each other and will sustain oscillation at the unity-gain
frequency.
The AC tables show φ
m
for a no load condition. But φ
m
changes with load. The Gain and Phase margin vs
Frequency plots in the curve section can be used to graphically determine the φ
m
for various loaded conditions.
To do this, examine the phase angle portion of the plot, find the phase margin point at the unity-gain frequency,
and determine how far this point is from zero degree of phase-margin. The larger the phase-margin, the more
stable the circuit operation.
The bandwidth is also affected by load. The graphs of Figure 32 and Figure 33 provide a quick look at how
various loads affect the φ
m
and the bandwidth of the LMV821/822/824 family. These graphs show capacitive
loads reducing both φ
m
and bandwidth, while resistive loads reduce the bandwidth but increase the φ
m
. Notice
how a 600Ω resistor can be added in parallel with 220 picofarads capacitance, to increase the φ
m
20°(approx.),
but at the price of about a 100 kHz of bandwidth.
Overall, the LMV821/822/824 family provides good stability for loaded condition.
Figure 32. Phase Margin vs Common Mode Voltage for Various Loads
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