Datasheet

1 +
R
F
R
IN
¨
¨
©
§
¨
¨
©
§
1 +
R
IN
|| R
F
R
C
¨
¨
©
§
¨
¨
©
§
= 18 dB = 7.9
LMP7717, LMP7718
www.ti.com SNOSAY7H –MARCH 2007–REVISED MARCH 2013
(9)
Now set R
F
= R
IN
= R. With these values and solving for R
C
we have R
C
= R/5.9. Note that the value of C does
not affect the ratio between the resistors. Once the value of the resistors is set, then the position of the pole in F
must be set. A 2 kΩ resistor is used for R
F
and R
IN
in this design. Therefore the value for R
C
is set at 330Ω, the
closest standard value for 2 kΩ/5.9.
Rewriting Equation 2 to solve for the minimum capacitor value gives the following equation:
C = 1/(2πf
p
R
C
) (10)
The feedback factor curve, F, intersects the A
VOL
curve at about 12 MHz. Therefore the pole of F should not be
any larger than 1.2 MHz. Using this value and R
C
= 330Ω the minimum value for C is 390 pF. Figure 51 shows
that there is too much overshoot, but the part is stable. Increasing C to 2.2 nF did not improve the ringing, as
shown in Figure 52.
Figure 51. First Try at Compensation, Gain = −1
Figure 52. C Increased to 2.2 nF, Gain = −1
Some over-compensation appears to be needed for the desired overshoot characteristics. Instead of intersecting
the A
VOL
curve at 18 dB, 2 dB of over-compensation will be used, and the A
VOL
curve will be intersected at 20
dB. Using Equation 5 for 20 dB, or 10 V/V, the closest standard value of R
C
is 240Ω. The following two
waveforms show the new resistor value with C = 390 pF and 2.2 nF. Figure 54 shows the final compensation and
a very good response for the 1 MHz square wave.
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