CS3011
CS3012
3.2.2
Gain Calculations Summary and
Recommendations
Verify the Opamp Compensation:
Verify the opamp co mpensation using the open-
loop gain and pha se resp onse Bode plot in
Figure 14. Plot the calculated clo sed loop gain
transfer function andverify the followingdesign cri-
teria are met:
Condition #1: |Av| ≤ 50 and R1 ≤ 100 Ω
The Opamp is inherently stable for |Av| ≤ 50 and
R1 ≤ 100 Ω . No C2 co mpensation cap acitor
across R2 is required.
•
Pole P1 > opamp in ternal 50 kHz c rossover fre-
quency
•
|Av| = 1 config uration ha s 7 0° pha se margin
and 20 dB gain margin.
-
-
P1 = 1 / [2π (R1||R2) • C2], where P1 = 1 MHz
To sim plify the ca lculation, set t he p ole t o
P1 = 1 MHz.
•
|Av| = 50 configuration ha s phase margin be-
tween 40° for C LOAD ≤ 100 pF and 60 ° for
CLOAD = 0 pF.
•
•
Z1 < opamp internal 50 kHz crossover frequency
Z1 = 1 / (2π R2 • C2)
-
Gain margin above the open-loop gain transfer
function is re quired. A g ain margin of +20 dB
above the open loop g ain transfer function is
optimal.
Condition #2: |Av| ≤ 50 and R1 > 100 Ω
Compensation capacitor C2 across R2 is required.
Calculate C2 using the following formula:
•
C2 ≥ (R1 • Cin) / R2, where Cin = 50 pF
Condition #3: |Av| > 50
Compensation capacitor C2 across R2 is required.
Calculate and verify a value for C2 using the fol-
lowing steps.
Calculate the Compensation Capacitor Value:
1) Calculate avalue forC2 using thefollowing for-
mula:
C2 = 1 / [2π (R1||R2) • P1], where P1 = 1 MHz
3.3
Powerdown (PDWN)
To simplify the calculation, set the pole of the filter
to P1 = 1 MHz. P1 must be set h igher than the
opamp’s internal 50 kHz crossover frequency.
The CS3011 single amplifier provides a power-
down function on pin 1. If this pin is left ope n the
amplifier will operate normally. If the powerdown is
asserted low, the amplifier enters a powered down
state. Th ere is a pu ll-up resistor (approximately
800 k ohm) inside the amplifier from pin 1 to the V+
supply. The current through this pull-up resistor is
the main source of current drain in the powerdown
state.
2) Calculate a second value for C2 using the fol-
lowing formula:
C2 ≥ (R1 • Cin) / R2, where Cin = 50 pF
3) Use the larger of the two values calculated in
steps 1 & 2.
DS597F6
13
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