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SSM2211_02

Low Distortion 1.5 Watt Audio Power Amplifier

Analog Devices 

SSM2211

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

must be determined by:

AV ,

MIN

= VSY

VTHS

(12)

Where, VSY is the single supply voltage and

VTHS is the threshold voltage.

AV should be set to a minimum of 2 for the circuit to work prop-

erly. Next choose R1 and set R2 to:

Ê 2ˆ

R2 = R1ËÁ1- AV ¯˜

(13)

Find R3 as:

( ) R3

=

R1¥

R1+

R2

R2

AV -1

(14)

C1 can be arbitrarily set but should be small enough to not cause

A2 to become capacitively overloaded. R4 and C1 will control the

shutdown rate. To prevent intermittent shutdown with low

frequency input signals, the minimum time constant should be:

10

R4 ¥ C1 ≥

(15)

fLOW

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 should 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

being used.

The minimum gain of the shutdown circuit from Equation 12 is

AV = 100. R1 is set to 100 kW, and using Equation 13 and

Equation 14, R2 = 98 kW and R3 = 4.9 MW. C1 is set to

0.01 mF, and based on Equation 15, R4 is set to 10 MW. To

minimize power supply current, R5 and R6 are set to 10 MW.

The above procedure will provide an adequate starting point for

the shutdown circuit. Some component values may need to be

adjusted empirically to optimize performance.

Turn On 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 sub-audible

range, greatly reducing startup popping noise. On power-up, the

midrail bypass capacitor is charged through an effective resis-

tance of 25 kW. To minimize start-up popping, the charging

time constant for CB should be greater than the charging time

constant for the input coupling capacitor, CC.

CB ¥ 25 kW > CC RI

(16)

For an application where R1 = 10 kW and CC = 0.22 mF, the

midrail bypass capacitor, CB, should be at least 0.1 mF to mini-

mize start-up popping noise.

SSM2211 Amplifier Design Example

Given:

Maximum Output Power

Input Impedance

Load Impedance

Input Level

Bandwidth

1W

20 kW

8W

1 V rms

20 Hz – 20 kHz ± 0.25 dB

The configuration shown in Figure 2 will be used. The first

thing to determine is the minimum supply rail necessary to ob-

tain the specified maximum output power. From Figure 6, for

1 W of output power into an 8 W load, the supply voltage must

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

from a voltage reference. The extra supply voltage will also al-

low the SSM2211 to reproduce peaks in excess of 1 W without

clipping the signal. With VDD = 5 V and RL = 8 W, Equation 9

shows that the maximum power dissipation for the SSM2211 is

633 mW. From the power derating curve in TPC 28, the ambi-

ent temperature must be less than 85∞C.

The required gain of the amplifier can be determined from

Equation 17:

AV

=

PL RL

VIN , rms

= 2.8

(17)

From Equation 1,

RF =

R1

AV

2

, or

RF

= 1.4 ¥ R1. Since the

desired input impedance is 20 kW, R1 = 20 kW and R2 = 28 kW.

The final design step is to select the input capacitor. Because

adding an input capacitor, CC, high pass filter, the corner frequency

needs to be far enough away for the design to meet the bandwidth

criteria. For a 1st order filter to achieve a passband response

within 0.25 dB, the corner frequency should be at least 4.14 times

away from the passband frequency. So, (4.14 ϫ fHP) < 20 Hz.

Using Equation 2, the minimum size of input capacitor can be found:

1

( ) CC

>

2p

20 kW

Ê 20 Hz ˆ

ËÁ 4.14 ¯˜

(18)

So CC > 1.65 mF. Using a 2.2 mF is a practical choice for CC.

The gain-bandwidth product for each internal amplifier in the

SSM2211 is 4 MHz. Because 4 MHz is much greater than

4.14 ؋ 20 kHz, the design will meet the upper frequency band-

width criteria. The SSM2211 could also be configured for higher

differential gains without running into bandwidth limitations.

Equation 16 shows an appropriate value for CB to reduce start-

up popping noise:

( )( ) 2.2 mF 20 kW

CB >

25 kW

= 1.76 mF

(19)

Selecting CB to be 2.2 mF for a practical value of capacitor will

minimize start-up popping noise.

–12–

REV. B

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