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LMV821M5 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LMV821M5 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 24 page APPLICATION NOTE This application note is divided into two sections: design considerations and Application Circuits. 1.0 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 1.1 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 φm changes with load. The Gain and Phase margin vs Fre- quency 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 1 and Figure 2 provide a quick look at how various loads af- fect 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 in- crease 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 band- width. Overall, the LMV821/822/824 family provides good stability for loaded condition. 1.2 Unity Gain Pulse Response Considerations A pull-up resistor is well suited for increasing unity-gain, pulse response stability. For example, a 600 Ω pull-up resis- tor reduces the overshoot voltage by about 50%, when driv- ing a 220 pF load. Figure 3 shows how to implement the pull-up resistor for more pulse response stability. Higher capacitances can be driven by decreasing the value of the pull-up resistor, but its value shouldn’t be reduced be- yond the sinking capability of the part. An alternate approach is to use an isolation resistor as illustrated in Figure 4 . Figure 5 shows the resulting pulse response from a LMV824, while driving a 10,000pF load through a 20 Ω isolation resistor. DS100128-60 FIGURE 1. Phase Margin vs Common Mode Voltage for Various Loads DS100128-61 FIGURE 2. Unity-Gain Frequency vs Common Mode Voltage for Various Loads DS100128-41 FIGURE 3. Using a Pull-up Resistor at the Output for Stabilizing Capacitive Loads DS100128-43 FIGURE 4. Using an Isolation Resistor to Drive Heavy Capacitive Loads www.national.com 11 |
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