LMV821 datasheet(11/24 Pages) NSC

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 (

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

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

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

m and the bandwidth of the LMV821/822/824 family.

These graphs show capacitive loads reducing both

m and

bandwidth, while resistive loads reduce the bandwidth but in-

crease the

m. Notice how a 600Ω resistor can be added in

parallel with 220 picofarads capacitance, to increase the

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

LMV821 Datasheet(PDF) 11 Page - National Semiconductor (TI)

LMV821 Datasheet(HTML) 11 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 21 22 23 24

Datasheet Download

Link URL