Cirrus Logic CS3002 - Manual (Page 11)

CS3002
DS490F10 11
3.2.2 Gain Calculation Recommendations
Condition 1: |Av|  50 and R1  100
The op amp is inherently stable for |Av|  50 and
R1  100. Capacitor C2 is not required for
compensation across resistor R2.
1) |Av| = 1 configuration has 70° phase margin and
20dB gain margin.
2) |Av| = 50 configuration has phase margin between
40° for C
LOAD
 100 pF and 60° for C
LOAD
 0pF.
Condition 2: |Av|  50 and R1  100
Compensation capacitor C2 across resistor R2 is
required. Calculate C2 using Equation 8:
where
C
in
= 50pF
Condition 3: |Av|  50
Compensation capacitor C2 across resistor R2 is
required. Calculate and verify a value for C2 using the
following steps.
Calculate the Compensation Capacitor Value:
1) Calculate a value for C2 using Equation 9:
where
P
1
= 1MHz
To simplify the calculation, set the pole of the filter to
P
1
= 1MHz. Pole P
1
must be set higher than the
op amp’s internal 50kHz crossover frequency.
2) Calculate a second value for C2 using Equation 10:
where
C
in
= 50pF
3) Use the larger of the two values calculated in steps
1 and 2.
Verify the Op Amp Compensation:
Verify the op amp compensation using the open-loop
gain and phase response Bode plot in
Figure 13. Plot
the calculated closed loop gain transfer function and
verify the following design criteria are met:
1) Pole P
1
> op amp internal 50kHz crossover
frequency
where
P
1
= 1MHz
To simplify the calculation, set the pole to P
1
= 1MHz.
2) Z1 < op amp internal 50kHz crossover frequency
3) Gain margin above the open-loop gain transfer
function is required. A gain margin of +20dB above
the open-loop gain transfer function is optimal.
C2
R1 C
in
  
R2
---------------------------  [Eq. 8]
C2
1
2  R1 R2    P
1
  
--------------------------------------------------- = [Eq. 9]
C2
R1 C
in
  
R2
---------------------------  [Eq. 10]
P
1
1
2  R1 R2    C2 
----------------------------------------------- = [Eq. 11]
Z
1
1
2  R2 C2   
---------------------------------- = [Eq. 12]