CS3002 datasheet(12/16 Pages) CIRRUS

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CS3001

CS3002

DS490F9

3.2.2

Gain Calculations Summary and

Recommendations

Condition #1: |Av|

≤ 50 and R1 ≤ 100 Ω

The Opamp is inherently stable for |Av|

≤ 50 and

≤ 100 Ω . No C2 compensation capacitor across

R2 is required.

•

|Av| = 1 configuration has 70° phase margin

and 20 dB gain margin.

•

|Av| = 50 configuration has phase margin be-

tween 40° for CLOAD ≤ 100 pF and 60° for

CLOAD = 0pF.

Condition #2: |Av|

≤ 50 and R1 > 100 Ω

Compensation capacitor C2 across R2 is required.

Calculate C2 using the following formula:

•C2

≥ (R1 • C

in) / 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 a value for C2 using the following

formula:

=1/ [2π (R1||R2) • P1], where P1 = 1 MHz

To simplify the calculation, set the pole of the filter

to P1 = 1 MHz. P1 must be set higher than the

opamp’s internal 50 kHz crossover frequency.

2) Calculate a second value for C2 using the fol-

lowing formula:

≥ (R1 • C

in) / R2, where Cin = 50 pF

3) Use the larger of the two values calculated in

steps 1 & 2.

Verify the Opamp Compensation:

Verify the opamp compensation using the open-

loop gain and phase response Bode plot in

Figure 15. Plot the calculated closed loop gain

transfer function and verify the following design

criteria are met:

•

Pole P1 > opamp internal 50 kHz crossover fre-

quency

P1=1/[2

π (R1||R2) • C2], where P1 = 1 MHz

To simplify the calculation, set the pole to

P1 = 1 MHz.

•

Z1 < opamp internal 50 kHz crossover frequency

Z1=1/(2

π R2 • C2)

•

Gain margin above the open-loop gain transfer

function is required. A gain margin of +20 dB

above the open loop gain transfer function is

optimal.

3.3

Powerdown (PDWN)

The CS3001 single amplifier provides a power-

down function on pin 1. If this pin is left open the

amplifier will operate normally. If the powerdown

is asserted low, the amplifier will go into a low

power state. There is a pull-up resistor (approxi-

mately 800 k

Ω) inside the amplifier from pin 1 to

the V+ supply. The current through this pull-up re-

sistor is the main source of current drain in the

powerdown state.

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