ssm2211z-eval Analog Devices, Inc., ssm2211z-eval Datasheet - Page 18

• Manufacturer Part Number: ssm2211z-eval
• Description: Low Distortion 1.5 Watt Audio Power Amplifier
• Manufacturer: Analog Devices, Inc.

SSM2211
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
where:
V
V
A
properly.
Next, choose R1 and set R2 to
Find R3 as
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
where f
SY
THS
V
must be set to a minimum of 2 for the circuit to work
is the single supply voltage.
is the threshold voltage.
A
R2
R3
R4
V,
LOW
×
=
MIN
=
C1
R1
R2
R1
is the lowest input frequency expected.
=
≥
⎛
⎜
⎜
⎝
×
+
1
V
V
R2
R2
f
−
THS
10
LOW
SY
A
2
(
V
A
V
⎞
⎟
⎟
⎠
−
1
)
(12)
(13)
(14)
(15)
Rev. D | Page 18 of 24
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 A
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
starts up. By adjusting the charging time constant of C
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
constant for the input coupling capacitor, C
For an application where R1 = 10 kΩ and C
be at least 0.1 μF to minimize start-up popping noise.
SSM2211 Amplifier Design Example
Maximum Output Power
Input Impedance
Load Impedance
Input Level
Bandwidth
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
V
C
= 100. R1 is set to 100 kΩ. Using Equation 13 and
B
× 25 kΩ > C
B
B
, determines the rate at which the SSM2211
C
needs to be greater than the charging time
× R
1
1 W
20 kΩ
8 Ω
1 V rms
20 Hz − 20 kHz ± 0.25 dB
C
C
.
= 0.22 μF, C
B
, the
B
must
(16)