FAB3103 Datasheet, PDF (7/13 Pages) Fairchild Semiconductor – 2.3 Watt Class-D Audio Amplifier with Integrated Boost Regulator and Automatic Gain Control

Detailed Description
Signal Path
The FAB3103 features a fully differential signal path for
noise rejection. The low-EMI design allows the OUT+
and OUT- pins to be connected directly to a speaker
without an output filter.
The input section includes an 80KHz low-pass filter for
removing out-of-band noise from audio sources, such as
sigma delta DACs.
Shutdown
If EN is grounded, the Class-D amplifier and the boost
regulator are turned off. IN+ and IN- are high
impedance. Audio signals present at IN+ and IN- with
amplitude less than the maximum differential input
voltage swing are not distorted by the FAB3103 (see
Electrical Characteristics).
When EN transitions from LOW to HIGH during the
wake-up time (see Electrical Characteristics), the
FAB3103 charges the input DC blocking capacitors to
the Common Mode voltage before enabling the Class-D
amplifier. To minimize click and pop during turn-on,
audio signals should not be present during the wake-up
period. Other devices that are connected to the same
input signal, if not muted, may experience a pop due to
this capacitor charging.
There is no limitation on the length of shutdown.
Remaining charge on the PVDD capacitor at startup (for
example, if EN is LOW for only a short period) does not
affect startup behavior.
The EN pin has an internal 300KΩ pull-down resistor.
EN must be LOW when V_BATT is lower than the V_BATT
under-voltage shutdown voltage (see Electrical
Characteristics). EN must remain LOW for at least
100µs after V_BATT rises above the V_BATT under-voltage
shutdown voltage.
Class-D Amplifier Over-Current Protection
If the output current of the Class-D amplifier exceeds
limits (see the Electrical Characteristics), the amplifier is
disabled for approximately one second. (Other systems,
such as the boost regulator and AGC, remain active.)
After one second, the amplifier is re-enabled. If the fault
condition still exists, the amplifier is disabled again. This
cycle repeats until the fault condition is removed.
Speaker Size
The FAB3103 was designed for use with small speakers
found in mobile applications. The back EMF in larger
speakers can cause PVDD to peak above safe levels.
To check safe operation, monitor PVDD while driving a
dynamic signal (such as music) at maximum levels. If
PVDD peaks above 6.2V, connect a 6V Zener diode
between PVDD and PGND.
Low EMI
To minimize EMI, edge-rate control for the boost
regulator and Class-D amplifier can be employed.
The boost regulator's edge-rate control is disabled by
default. For devices with 20ns boost edge rates or 10ns
boost edge rates, contact a Fairchild Representative.
This is a factory option that cannot be changed in the
application, but is available from Fairchild.
The Class-D amplifier's edge-rate control is disabled by
default. For devices with 20ns Class-D edge rates,
contact a Fairchild Representative. This is a factory
option that cannot be changed in the application, but is
available from Fairchild..
Automatic Boost Shutdown
Automatic boost shutdown changes the Class-D
amplifier supply voltage as a function of audio output
level. At audio output levels above 2V_pk, the boost
converter generates 5.65V from the input battery
voltage. If the output level is below 2V_pk for more than
125ms, the boost converter is switched off and the
Class-D amplifier is supplied directly from the battery.
As a result, efficiency is improved at low audio output
levels and quiescent current consumption is reduced.
Figure 9 shows an example of an auto boost startup
event. At first, the boost converter is off and PVDD is
the same voltage as VBATT. At 20µs, a large audio
signal is presented at the inputs, which causes the
boost converter to start up. From 20µs to 120µs, battery
current is ramped up. The auto boost startup current
ramp rate is 15mA/µs. This ramp is enforced to avoid
sudden current draw spikes from the battery.
At 120µs, after PV_DD has reached the Boost Converter
Output Voltage, the ramp is released and battery current
falls to a level capable of sustaining the speaker
amplifier’s outputs. At 160µs, the input signal begins to
rise, which increases battery current. At 180µs, the
boost converter peak input current limit is enforced and
battery current levels off, which causes PV_DD to droop.
The boost regulator should not be used to drive any
loads other than the Class-D amplifier.
© 2011 Fairchild Semiconductor
FAB3103 • Rev. 1.0.2
7
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