Datasheet
Data Sheet SSM2211
Rev. G | Page 19 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. The diode, 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 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 the AD8500 or equivalent. This
ensures that C1 discharges sufficiently to bring the SSM2211
out of shutdown mode.
To find the appropriate component values, the gain of A2 must
be determined by
THS
SY
MINV,
V
V
A
(12)
where:
V
SY
is the single supply voltage.
V
THS
is the threshold voltage.
A
V
must be set to a minimum of 2 for the circuit to work
properly.
Next, choose R1 and set R2 to
V
A
R1R2
2
1
(13)
Find R3 as
1
V
A
R2R2
R2R1
R3
(14)
C1 can be arbitrarily set but must 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
f
C1R4
10
(15)
where
f
LOW
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 must 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
A
V
= 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, C
B
, determines the rate at which the SSM2211
starts up. By adjusting the charging time constant of C
B
, 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 C
B
must be greater than the charging time constant for the
input coupling capacitor, C
C
.
C
B
× 25 kΩ > C
C
× R1 (16)
For an application where R1 = 10 kΩ and C
C
= 0.22 μF, C
B
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
voltage reference. The extra supply voltage also allows the
SSM2211 to reproduce peaks in excess of 1 W without clipping
the signal. With V
DD
= 5 V and R
L
= 8 Ω, Equation 9 shows that
the maximum power dissipation for the SSM2211 is 633 mW.
From the power derating curve in Figure 31, the ambient
temperature must be less than 50°C for the SOIC and 121°C for
the LFCSP.
The required gain of the amplifier can be determined from
Equation 17 as
8.2
rmsIN,
LL
V
V
RP
A
(17)
From Equation 1,
2
V
I
F
A
R
R
or R
F
= 1.4 × R
I
. Because the desired input impedance is 20 kΩ,
R
I
= 20 kΩ and R2 = 28 kΩ.