1 +

RF

RIN

¨

¨

©

§

1 +

RIN || RF

RC

¨

¨

©

§

= 18 dB = 7.9

LMP7717, LMP7718

www.ti.com

SNOSAY7H – MARCH 2007 – REVISED MARCH 2013

(9)

Now set RF = RIN = R. With these values and solving for RC we have RC = 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 k

Ω resistor is used for RF and RIN in this design. Therefore the value for RC is set at 330Ω, the

closest standard value for 2 k

Ω/5.9.

Rewriting Equation 2 to solve for the minimum capacitor value gives the following equation:

C = 1/(2

πfpRC)

(10)

The feedback factor curve, F, intersects the AVOL curve at about 12 MHz. Therefore the pole of F should not be

any larger than 1.2 MHz. Using this value and RC = 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 AVOL curve at 18 dB, 2 dB of over-compensation will be used, and the AVOL curve will be intersected at 20

dB. Using Equation 5 for 20 dB, or 10 V/V, the closest standard value of RC 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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