FAB3103 Datasheet, PDF (8/13 Pages) Fairchild Semiconductor – 2.3 Watt Class-D Audio Amplifier with Integrated Boost Regulator and Automatic Gain Control

0.8
0.6
0.4
0.2
0
0
20 40 60 80 100 120 140 160 180 200
6.0
4.0
2.0
0.0
0
20 40 60 80 100 120 140 160 180 200
2.5
2.0
Boost Converter Peak Input Current Limit
1.5
1.0
0.5
0.0
0
20 40 60 80 100 120 140 160 180 200
Time (µs)
Figure 9. Auto Boost Startup
Automatic Gain Control
Due to constant output power, the amount of VBATT
current needed to maintain a given output amplitude is
inversely proportional to VBATT voltage. This produces
very large current requirements at low V_BATT. The AGC
eases low-V_BATT current demands by reducing the gain
when VBATT voltage drops below a trip point. One of
three different trip points may be selected by shorting
AGCT to VBATT, shorting AGCT to PGND, or floating
AGCT (see Electrical Characteristics).
The trip point is determined upon power-on and when
EN transitions from LOW to HIGH. If AGCT is changed
during operation, the new value is not read until power
or EN is cycled.
When V_BATT is above the trip point, the AGC has no
effect on the signal path.
When V_BATT is at or below the trip point, target gain is
reduced in 0.5dB steps according to the equation:
G_{t arg et}

G_I

S
L^GI

V_T 
V_out
V_batt
max

(1)
where:
G_I
= Initial gain (10V/V);
S_L
= 3V/V slope;
V_OUTMAX = 5.2V;
V_T
= AGC trip point set by the AGCT pin; and
V_BATT = Voltage at the VBATT pin.
Target gain can be reduced by as much as 10dB.
Note that the state of auto boost shutdown has no effect
on the AGC.
Figure 10 shows target gain vs. battery voltage.
© 2011 Fairchild Semiconductor
FAB3103 • Rev. 1.0.2
8
www.fairchildsemi.com