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参数资料
| 型号： | LM4918LQ/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 音频控制 |
| 英文描述： | 2 CHANNEL(S), VOLUME CONTROL CIRCUIT, QCC32 |
| 封装： | LLP-32 |
| 文件页数： | 4/17页 |
| 文件大小： | 646K |
| 代理商： | LM4918LQ/NOPB |
Application Information (Continued)
AUTOMATIC GAIN CONTROL (AGC)
The AGC represents a closed loop gain control network in
conjunction with the peak detect monitoring the outputs of
the power amplifier section. A reference voltage is put on the
“AGC Ref In” pin to set the desired dynamic range of the
AGC. This voltage should be set to allow for maximum
desired output level for the expected range of the input
signal. The AGC may provide anywhere from 0 to 24dB of
attenuation. The peak detect constantly updates the AGC
block with information about the maximum level of the output
signal. Internally, if this peak value is higher than the set
reference voltage, the AGC will reduce the gain of the circuit
until the output peak is lower than the reference voltage. This
entire process is timed by the Chold capacitor (in conjunction
with Rdischarge), which determines the hold time between
peaks while restoring gain (gain returns to normal levels
after being attenuated by large power amp output signal).
The circuit shown in Figure 3 best describes the AGC loop
operation. Square nodes are external pins. Vbp is the by-
pass voltage. As is shown below, the peak detect section
(PD) monitors all outputs of the power amp (VoL+, VoL-,
VoR+, and VoR-). Peaks are fed into the peak buffer, regu-
lated by Chold and Rdis. The output of the peak buffer
(VpkBfr) is then compared to VagcRef. The AGC amplifier
then sets the ADC output to control the gain setting in 32
discrete steps from 0 to –24dB. Gain will remain reduced
until peak levels (Vpkbfr) return below the VagcRef, as buff-
ered by the peak buffer and Chold. This means gain will
slowly increase (speed determined by Chold) back to normal
levels (maximum gain set) or until the peaks again exceed
the reference level.
The reference voltage VagcRef should be set in conjunction
with the gain settings on the power amplifier stage to provide
maximum gain (0dB) for the typical desired maximum output
level of the part for a maximum input level. This will ensure
that both the full 24dB dynamic range of the AGC is used
effectively and that maximum effective audio levels may be
reached without clipping. Typical AGC peak gain values for
given VagcRef voltages are shown in Table 1. These are
taken with a 20M
pullup resistor on Chold (as used shown
on the demo board). These are shown as typical values only
– VagcRef level should be set with the factors described
above.
200913E7
FIGURE 3. Block Diagram with external component for AGC closed loop
LM4918
www.national.com
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