LM8261 datasheet(16/19 Pages) NSC

16 / 19 page

Application Notes: (Continued)

Op Amp’s compensation capacitor value and available cur-

rent into that capacitor. Beyond 10nF, the Slew Rate is deter-

mined by the Op Amp’s available output current. Note that

because of the lower output sourcing current compared to

the sinking one, the Slew Rate limit under heavy capacitive

loading is determined by the positive transitions. An estimate

of positive and negative slew rates for loads larger than

100nF can be made by dividing the short circuit current value

by the capacitor.

For the LM8261, the available output current increases with

the input overdrive. Referring to

Figure 3 and Figure 4, Out-

put Short Circuit Current vs. Input Overdrive, it can be seen

that both sourcing and sinking short circuit current increase

as input overdrive increases. In a closed loop amplifier con-

figuration, during transient conditions while the fed back out-

put has not quite caught up with the input, there will be an

overdrive imposed on the input allowing more output current

than would normally be available under steady state condi-

tion. Because of this feature, the Op Amp’s output stage qui-

escent current can be kept to a minimum, thereby reducing

power consumption, while enabling the device to deliver

large output current when the need arises (such as during

transients).

Figure 5 shows the output voltage, output current, and the

resulting input overdrive with the device set for A

V = +1 and

the input tied to a 1V

pp step function driving a 47nF capaci-

tor. As can be seen, during the output transition, the input

overdrive reaches 1V peak and is more than enough to

cause the output current to increase to its maximum value

(see

Figure 3 and Figure 4 plots). Note that because of the

larger output sinking current compared to the sourcing one,

the output negative transition is faster than the positive one.

Estimating the output voltage swing:

It is important to keep in mind that the steady state output

current will be less than the current available when there is

an input overdrive present. For steady state conditions, the

Output Voltage vs. Output Current plot (Typical Performance

Characteristics section) can be used to predict the output

swing.

Figure 6 and Figure 7 show this performance along

with several load lines corresponding to loads tied between

the output and ground. In each cases, the intersection of the

device plot at the appropriate temperature with the load line

would be the typical output swing possible for that load. For

example, a 1K

Ω load can accomadate an output swing to

within 250mV of V

− and to 330mV of V+ (Vs = +/−15V) cor-

responding to a typical 29.3V

pp unclipped swing.

DS101084-57

FIGURE 3. Output Short Circuit Sourcing Current vs

Input Overdrive

DS101084-56

FIGURE 4. Output Short Circuit Sinking Current vs

Input Overdrive

DS101084-39

FIGURE 5. Buffer Amplifier scope photo

DS101084-60

FIGURE 6. Output Sourcing Characteristics with Load

Lines

LM8261 Datasheet(PDF) 16 Page - National Semiconductor (TI)

LM8261 Datasheet(HTML) 16 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

Datasheet Download

Link URL