SSM2211_04 데이터 시트보기 (PDF) - Analog Devices
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SSM2211_04 Datasheet PDF : 20 Pages
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The input signal to the SSM2211 is also connected to the non-
inverting terminal of A2. R1, R2, and R3 set the threshold
voltage for when 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. D1 should be a 1N914 or equivalent diode and A2
should be a rail-to-rail output amplifier, such as an 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
AV,MIN
= VSY
VTHS
(12)
where:
VSY is the single supply voltage.
VTHS is the threshold voltage.
AV should be set to a minimum of 2 for the circuit to work
properly.
Next choose R1 and set R2 to
R2
=
R1⎜⎜⎝⎛1 −
2
AV
⎟⎟⎠⎞
(13)
Find R3 as:
R3
=
R1 ×
R2 +
R2
R2
(AV
− 1)
(14)
C1 can be arbitrarily set but should be small enough to keep 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 should be
R4 × C1 ≥ 10
(15)
f LOW
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 used.
SSM2211
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 should be greater than the charging time
constant for the input coupling capacitor, CC.
CB × 25 kΩ > CCR1 (16)
For an application where R1 = 10 kΩ and CC = 0.22 µF, the
midrail bypass capacitor, CB, should 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
1W
20 kΩ
8Ω
1 V rms
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 VDD = 5 V and RL = 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 73°C for the SOIC and 118°C for
the LFCSP.
The required gain of the amplifier can be determined from
Equation 17 as
AV
=
PL RL
VIN,rms
= 2.8
(17)
Rev. C | Page 17 of 20
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