FAB3103 Datasheet, PDF (9/13 Pages) Fairchild Semiconductor – 2.3 Watt Class-D Audio Amplifier with Integrated Boost Regulator and Automatic Gain Control

10.0
9.5
89..50
8.0
7.5
7.0
66..05
5.5
5.0
44..05
3.5
3.0
2.5 3.0 3.5 4.0 4.5 5.0
V_BATT (V)
Line Color
Red
Green
Blue
AGCT Configuration
Float
Ground
V_BATT
AGC Trip Point (V)
3.25
3.55
3.75
Figure 10. Target Gain vs. Battery Voltage
Figure 11 is similar to Figure 10 except that the target
gain is expressed in dB rather than V/V.
20
18
16
14
12
10
8
2.5 3.0 3.5 4.0 4.5 5.0
V_BATT (V)
Line Color
Red
Green
Blue
AGCT Configuration
Float
Ground
V_BATT
AGC Trip Point (V)
3.25
3.55
3.75
Figure 11. Target Gain vs. Battery Voltage
Figure 12 shows examples of peak output voltage vs.
battery voltage.
3.5
3
2.5
2
V_IN = 0.3V_pk
1.5
1
0.5
2.5 3.0 3.5 4.0 4.5 5.0
V_BATT (V)
Line
Color
Magenta
Cyan
Black
AGCT
Configuration
Float
Ground
V_BATT
AGC Trip
Point (V)
3.25
3.55
3.75
Input Voltage
(V_pk)
0.3
0.3
0.3
Figure 12. Output Voltage vs. Battery Voltage
Figure 13 shows examples of output power vs. battery
voltage with a 0.4V_pk sinusoidal input signal.
1.0
0.8
0.6
0.4
R_L = 8Ω + 33µH
0.2
0.0
2.5
3.0 _VB3_AT._T5_(V) 4.0
4.5
5.0
Line
Color
Magenta
Cyan
Black
AGCT
Configuration
Float
Ground
V_BATT
AGC Trip
Point (V)
3.25
3.55
3.75
Input Voltage
(V_pk)
0.3
0.3
0.3
Figure 13. Output Power vs. Battery Voltage
Examples (V_IN=0.4V_pk Sine)
The speed at which gain can change is limited (see
Electrical Characteristics); therefore, the actual gain
may lag the target gain if V_BATT voltage changes quickly.
Figure 14 and Figure 15 show examples of AGC
changes over time. In these examples, AGCT is
grounded, so the AGC trip point is 3.55V.
1. Initially, V_BATT is 3.6V and gain is 10V/V (20dB).
2. A narrow V_BATT drop of less than 2µs is ignored by
the AGC.
3. The next V_BATT drop lasts longer and the AGC is
tripped. The initial 0.5dB gain reduction occurs
3.9µs after V_BATT crosses below the 3.55V trip point.
4. V_BATT is now 3.1V, so target gain is 10V/V – 3V/V ×
10V/V × [(3.55V – 3.1V) / 5.2V]=7.40V/V=17.4dB.
5. Gain continues to drop by 0.5dB every 10µs until it
is below the target gain, where it settles at 17.0dB.
6. When V_BATT rises above the trip point, gain
increases by 0.5dB. If more than 800ms has
passed since the last gain change, gain rises
immediately, as shown in Figure 14. Otherwise,
gain does not rise until after 800ms has passed, as
shown in Figure 15.
7. While V_BATT remains above the trip point, gain
continues to increase by 0.5dB every 800ms until it
returns to 20dB.
The intent of the AGC circuitry is to limit current draw
from the battery to extend runtime. This is particularly
important for handsets that incorporate advanced
shutdown algorithms to measure battery voltage. The
AGC circuit dynamically adjusts the amplifier gain based
on the trip point used. Even though the amplifier gain is
reduced in response to lower battery voltages, two
conditions result in continued higher current draw: 1) the
handset volume is turned up in an attempt to maintain
the same loudness, or 2) the input signal is increased. If
© 2011 Fairchild Semiconductor
FAB3103 • Rev. 1.0.2
9
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