LMP7717MAX NSC [National Semiconductor], LMP7717MAX Datasheet - Page 16

• Manufacturer Part Number: LMP7717MAX
• Description: 88 MHz, Precision, Low Noise, 1.8V CMOS Input, Decompensated Operational Amplifier
• Manufacturer: NSC [National Semiconductor]

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Some over-compensation appears to be needed for the de-
sired overshoot characteristics. Instead of intersecting the
A
and the A
tion 5 for 20 dB, or 10 V/V, the closest standard value of R
is 240Ω. The following two waveforms show the new resistor
value with C = 390 pF and 2.2 nF. Figure 9 shows the final
compensation and a very good response for the 1 MHz
square wave.
VOL
FIGURE 6. First Try at Compensation, Gain = −1
FIGURE 8. R
curve at 18 dB, 2 dB of over-compensation will be used,
FIGURE 7. C Increased to 2.2 nF, Gain = −1
VOL
curve will be intersected at 20 dB. Using Equa-
C
= 240Ω and C = 390 pF, Gain = −1
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C
16
To summarize, the following steps were taken to compensate
the LMP7717 for a gain of −1:
1.
2.
3.
The LMP7715 is the fully compensated part which is compa-
rable to the LMP7717. Using the LMP7715 in the same setup,
but removing the compensation network, provided the re-
sponse shown in Figure 10 .
For large signal response the rise and fall times are dominat-
ed by the slew rate of the op amps. Even though both parts
are quite similar the LMP7717 will give rise and fall times
about 2.5 times faster than the LMP7715. This is possible
because the LMP7717 is a decompensated op amp and even
though it is being used at a gain of −1, the speed is preserved
by using a good technique for external compensation.
Values for R
to give an expected phase margin of 45°. The values
were based on R
R
To reduce the ringing C was increased to 2.2 nF which
moved the pole of F, the feedback factor, farther away
from the A
There was still too much ringing so 2 dB of over-
compensation was added to F. This was done by
decreasing R
FIGURE 9. R
c
= 330Ω and C = 390 pF.
FIGURE 10. LMP7715 Response
VOL
c
C
C
and C were calculated from the Bode plot
curve.
to 240Ω.
= 240Ω and C = 2.2 nF, Gain = −1
IN
= R
F
= 2 kΩ. These calculations gave
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