FAB2200 Datasheet, PDF (15/33 Pages) Fairchild Semiconductor – Audio Subsystem with Stereo Class-G Headphone Amplifier and 1.2W Mono Class-D Speaker Amplifier

Programmable Headphone Amplifier
Noise Gate
The headphone noise gate automatically mutes the
headphone amplifier when its input amplitudes are low
to reduce noise during inactivity. This function is not
recommended for music playback, but is effective for
speech. The amplitude is measured after input pre-
amplifiers and before the headphone amplifier volume
controls. The headphone noise gate threshold level is
set by the HPNGTHRESH register. The amplitudes of
both channels must be less than the noise gate
threshold for a time determined by the HPNGTIME
register. When the noise gate mutes the amplifier, the
HPNGTRIP bit is set to 1.
If either channel’s input amplitude goes above the
headphone noise gate threshold, both amplifiers are un-
muted and the HPNGTRIP bit is set to 0. The amplifiers
are returned to the former HPxVOL values.
If either channel is in High-Impedance Mode (HIZx=1),
all inputs to that headphone should be deselected
(SELHPxx=0) so the noise gate ignores the HIZ channel.
If the HPNGZRA bit is set to 0, the headphone noise
gate attack (mute) function occurs immediately rather
than waiting for zero-crossing detection or ramping. If
the HPNGZRA bit is set to 1, the headphone noise gate
attack function obeys headphone zero-crossing
detection and ramp settings.
If the HPNGZRR bit is set to 0, the headphone noise
gate release (un-mute) function occurs immediately
rather than waiting for zero-crossing detection or
ramping. If the HPNGZRR bit is set to 1, the headphone
noise gate release (un-mute) function obeys headphone
zero crossing detection and ramp settings.
Table 5. Headphone Noise Gate Threshold
Voltage
HPNGTHRESH
[2:0]
Noise Gate Threshold (mV_pk)
000
Headphone Noise Gate Disabled
001
2.8
010
5.7
011
11.3
100
22.6
101
45.3
110
90.5
111
181.0
Table 7. Headphone Noise Gate Timing
HPNGTIME [2:0]
Time (ms)
000
10
001
20
010
40
011
80
100
160
HPNGTIME [2:0]
101
110
111
Time (ms)
320
640
Reserved
Certain combinations of HPRAMP, HPZCD, HPNGZRA,
and HPNGZRR are valid as shown in Table 8.
Combinations not listed may produce unpredictable
results (X = don’t care).
Table 8. Valid Headphone Amplifier Ramp /
Zero Crossing / Noise Gate Combinations
HPRAMP HPZCD HPNGZRA HPNGZRR
0
0
X
X
X
X
1
1
1
X
1
0
Class-D Speaker Amplifier
The class-D amplifier achieves greater than 90% efficiency.
Programmable spread spectrum and edge rate control
minimize electromagnetic interference (EMI). Rise and fall
times are limited to 20ns per transition at all power levels.
Programmable Automatic Gain Control
(AGC)
The speaker amplifier’s AGC can be used to limit output
amplitude and reduce clipping as supply voltage varies.
The AGC allows high-volume settings while minimizing
distortion and protecting the speaker element.
AGC works by comparing the threshold voltage against
a proposed output amplitude (the signal’s amplitude
after all gain stages, before the PWM modulator). If the
threshold is exceeded, gain is dynamically reduced until
the output voltage level no longer exceeds the threshold
or the minimum gain setting. When the output voltage
level no longer exceeds the threshold, gain is slowly
increased until either the output voltage level exceeds
the threshold again or the starting gain is reached.
AGC settings should not be changed while the speaker
amplifier is on. Before making changes to THMAX,
THVBATT, AGCATTACK, AGCRELEASE, or AGCMIN;
the speaker amplifier should be turned off by clearing all
SELSPAn bits to 0 or clearing SHDNB to 0.
AGC Threshold
The AGC threshold can be thought of as a target for the
maximum output amplitude. It is defined by the THMAX
and THVBATT registers.
THMAX defines the maximum threshold value
regardless of V_BATT supply voltage. This is useful for
protecting speakers from high amplitudes. Table 9
shows the THMAX threshold settings as well as the
corresponding maximum RMS power (assuming a 1KHz
sine wave into an 8 load).
© 2009 Fairchild Semiconductor Corporation
FAB2200 • Rev. 1.0.1
15
www.fairchildsemi.com