Datasheet
1 +
R
F
R
IN
¨
¨
©
§
¨
¨
©
§
1 +
R
IN
|| R
F
R
C
¨
¨
©
§
¨
¨
©
§
= 18 dB = 7.9
1k 10k 100k 1M 10M 100M
-20
0
20
40
60
80
100
GAIN (dB) and PHASE (°C)
FREQUENCY (Hz)
PHASE
A
VOL
with ADDITIONAL
COMPENSATION
GBP
2
nd
POLE
ADDITIONAL
COMPENSATION
45° PHASE
MARGIN
f
2
F
1
F
1
LMV793, LMV794
SNOSAX6D –MARCH 2007–REVISED MARCH 2013
www.ti.com
Figure 45. LMV793/LMV794 Simplified Bode Plot
To obtain stable operation with gains under 10 V/V the open loop gain margin must be reduced at high
frequencies to where there is a 45° phase margin when the gain margin of the circuit with the external
compensation is 0 dB. The pole and zero in F, the feedback factor, control the gain margin at the higher
frequencies. The distance between F and A
VOL
is the gain margin; therefore, the unity gain point (0 dB) is where
F crosses the A
VOL
curve.
For the example being used R
IN
= R
F
for a gain of −1. Therefore F = 6 dB at low frequencies. At the higher
frequencies the minimum value for F is 18 dB for 45° phase margin. From Equation 8 we have the following
relationship:
(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 are 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 5 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 46 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 47.
Figure 46. First Try at Compensation, Gain = −1
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