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

Ω 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

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