FAB2210 Datasheet, PDF (13/35 Pages) Fairchild Semiconductor – Audio Subsystem with Class-G Headphone and 3.3W Mono Class-D Speaker with Dynamic Range Compression

Charge Pump
The FAB2210 includes an inverting charge pump that
generates HVDD and HVSS (the headphone amplifier
power supplies) from the DVDD power supply input. The
HVSS rail is a negative mirror of HVDD and allows the
headphone amplifier to be ground referenced. The
ground-referenced biasing scheme allows the
headphone amplifier outputs to be biased at ground
while operating from a single external supply. This
eliminates the need for the large, expensive, DC-
coupling capacitors between the headphone amplifier
output and load that are required on traditional, single-
supply, V_DD/2, biased headphone amplifiers.
The negative HVSS rail allows the input preamplifiers to
be ground referenced. Input DC-blocking capacitors are
still required at INA1, INA2, INB1, and INB2 if the audio
source driving the input preamplifiers is biased above
ground. The input DC-blocking capacitors are not
required if the audio source driving the input
preamplifiers is also ground referenced and does not
present any DC offset to the FAB2210.
Class-G Operation
Compared with a traditional Class-AB amplifier, the
FAB2210’s Class-G architecture reduces power
consumption and extends battery life during headphone
playback. The power supply rails (HVDD and HVSS) of
the Class-G headphone amplifier adapt to the level of
audio signal present at the output. The adaptive nature
of the power supply rails ensures that energy is not
wasted during quiet passages of music or when the
volume of the headphone path is reduced.
During stereo headphone playback, when the
headphone output amplitude is below the V_TH threshold
level of 250mV_pk (typical); the charge pump efficiently
divides V_DVDD so that V_HVSS = -V_DVDD/2 and V_HVDD =
V_DVDD/2. When the headphone output amplitude
exceeds 250mV_pk (typical), the charge pump generates
higher magnitude rails, where V_HVSS=-V_DVDD and V_HVDD =
V_DVDD to allow for higher output amplitudes. Due to the
high crest factor of music and speech, a significant
portion of the audio content is below the V_TH threshold,
even at typical volume level settings for headphone
playback. When operating at the lower magnitude rails,
less power is dissipated within the headphone amplifier.
The transition from the lower magnitude rails to the
higher magnitude rails occurs fast enough to prevent
audible artifacts during headphone playback. The
transition from the higher magnitude rails to the lower
magnitude rails occurs 15ms (typical) from the last
threshold crossing (low to high) and only after the
headphone output amplitude has stayed below the
threshold level. The 15ms (typical) hysteresis prevents
the headphone amplifier power rails from bouncing
between high and low rails when the audio signal
approaches the threshold level.
Due to the flexible input to output routing capabilities of
the FAB2210, additional logic added to the Class-G
audio-level-detection circuit ensures that enough
headroom is available to avoid saturating the audio
input signals at the INA1, INA2, INB1, and INB2
preamplifiers. This is especially useful in cases where
both the Class-D amplifier path and/or mono headphone
path are selected for playback.
Figure 24. Class-G Headphone Amplifier Power Supply Rail Operation
© 2011 Fairchild Semiconductor Corporation
FAB2210 • Rev. 1.1.1
13
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