SSM2211CP-R2 datasheet(18/24 Pages) AD

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SSM2211

Rev. D | Page 18 of 24

The input signal to the SSM2211 is also connected to the non-

inverting terminal of A2. R1, R2, and R3 set the threshold

voltage at which the SSM2211 is to be taken out of shutdown

mode. D1 half-wave rectifies the output of A2, discharging C1

to ground when an input signal greater than the set threshold

voltage is detected. R4 controls the charge time of C1, which

sets the time until the SSM2211 is put back into shutdown

mode after the input signal is no longer detected.

R5 and R6 are used to establish a voltage reference point equal

to half of the supply voltage. R7 and R8 set the gain of the

SSM2211. A 1N914 or equivalent diode is required for D1, and

A2 must be a rail-to-rail output amplifier, such as OP181 or

equivalent. This ensures that C1 discharges sufficiently to bring

the SSM2211 out of shutdown mode.

To find the appropriate component values, first, the gain of A2

must be determined by

THS

MIN

(12)

where:

VSY is the single supply voltage.

VTHS is the threshold voltage.

AV must be set to a minimum of 2 for the circuit to work

properly.

Next, choose R1 and set R2 to

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

(13)

Find R3 as

(

−

)

(14)

C1 can be arbitrarily set but should be small enough to prevent

A2 from becoming capacitively overloaded. R4 and C1 control

the shutdown rate. To prevent intermittent shutdown with low

frequency input signals, the minimum time constant must be

LOW

≥

(15)

where fLOW is the lowest input frequency expected.

SHUTDOWN-CIRCUIT DESIGN EXAMPLE

In this example, a portable radio application requires the

SSM2211 to be turned on when an input signal greater than

50 mV is detected. The device needs to return to shutdown mode

within 500 ms after the input signal is no longer detected. The

lowest frequency of interest is 200 Hz, and a 5 V supply is used.

The minimum gain of the shutdown circuit, from Equation 12,

is AV = 100. R1 is set to 100 kΩ. Using Equation 13 and

Equation 14, R2 = 98 kΩ and R3 = 4.9 MΩ. C1 is set to 0.01 μF,

and based on Equation 15, R4 is set to 10 MΩ . To minimize

power supply current, R5 and R6 are set to 10 MΩ. The

previous procedure provides an adequate starting point for the

shutdown circuit. Some component values may need to be

adjusted empirically to optimize performance.

START-UP POPPING NOISE

During power-up or release from shutdown mode, the midrail

bypass capacitor, CB, determines the rate at which the SSM2211

starts up. By adjusting the charging time constant of CB, the

start-up pop noise can be pushed into the subaudible range,

greatly reducing start-up popping noise. On power-up, the

midrail bypass capacitor is charged through an effective

resistance of 25 kΩ. To minimize start-up popping, the charging

time constant for CB needs to be greater than the charging time

constant for the input coupling capacitor, CC.

CB × 25 kΩ > CC × R1

(16)

For an application where R1 = 10 kΩ and CC = 0.22 μF, CB must

be at least 0.1 μF to minimize start-up popping noise.

SSM2211 Amplifier Design Example

Maximum Output Power

1 W

Input Impedance

20 kΩ

Load Impedance

8 Ω

Input Level

1 V rms

Bandwidth

20 Hz − 20 kHz ± 0.25 dB

The configuration shown in Figure 42 is used. The first thing to

determine is the minimum supply rail necessary to obtain the

specified maximum output power. From Figure 46, for 1 W of

output power into an 8 Ω load, the supply voltage must be at

least 4.6 V. A supply rail of 5 V can be easily obtained from a

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