Datasheet

=
1
F
f =
f
1 +
R
F
R
IN
¨
¨
©
§
¨
¨
©
§
1 +
R
IN
|| R
F
R
C
¨
¨
©
§
¨
¨
©
§
=
1
F
f = 0
1 +
R
F
R
IN
f
z
=
1
2SR
c
+ R
IN
|| R
F
)C
f
p
=
1
2SR
c
C
=
1
F
¨
¨
©
§
1 +
R
F
R
IN
¨
¨
©
§
¨
¨
©
§
1 + s(R
c
+ R
IN
|| R
F
) C
1 + sR
c
C
¨
¨
©
§
LMV793, LMV794
www.ti.com SNOSAX6D –MARCH 2007–REVISED MARCH 2013
(4)
where 1/F's pole is located at
(5)
1/F's zero is located at
(6)
(7)
The circuit gain for Figure 44 at low frequencies is −R
F
/R
IN
, but F, the feedback factor is not equal to the circuit
gain. The feedback factor is derived from feedback theory and is the same for both inverting and non-inverting
configurations. Yes, the feedback factor at low frequencies is equal to the gain for the non-inverting configuration.
(8)
From this formula, we can see that
• 1/F's zero is located at a lower frequency compared with 1/F's pole.
• 1/F's value at low frequency is 1 + R
F
/R
IN
.
• This method creates one additional pole and one additional zero.
• This pole-zero pair will serve two purposes:
– To raise the 1/F value at higher frequencies prior to its intercept with A, the open loop gain curve, in order
to meet the G
min
= 10 requirement. For the LMV793/LMV794 some overcompensation will be necessary
for good stability.
– To achieve the previous purpose above with no additional loop phase delay.
Please note the constraint 1/F ≥ G
min
needs to be satisfied only in the vicinity where the open loop gain A and
1/F intersect; 1/F can be shaped elsewhere as needed. The 1/F pole must occur before the intersection with the
open loop gain A.
In order to have adequate phase margin, it is desirable to follow these two rules:
1. 1/F and the open loop gain A should intersect at the frequency where there is a minimum of 45° of phase
margin. When over-compensation is required the intersection point of A and 1/F is set at a frequency where
the phase margin is above 45°, therefore increasing the stability of the circuit.
2. 1/F’s pole should be set at least one decade below the intersection with the open loop gain A in order to take
advantage of the full 90° of phase lead brought by 1/F’s pole which is F’s zero. This ensures that the effect of
the zero is fully neutralized when the 1/F and A plots intersect each other.
Calculating Lead-Lag Compensation for LMV793/LMV794
Figure 45 is the same plot as Figure 41, but the A
VOL
and phase curves have been redrawn as smooth lines to
more readily show the concepts covered, and to clearly show the key parameters used in the calculations for
lead-lag compensation.
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