TFA9891 All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2016. All rights reserved.
Product short data sheet Rev. 1 — 21 December 2016 8 of 24
NXP Semiconductors
TFA9891
9.5 V boosted audio system with adaptive sound maximizer and
speaker protection
8. Functional description
The TFA9891 is a highly efficient mono Bridge Tied Load (BTL) class-D audio amplifier
with a sophisticated SpeakerBoost protection algorithm. Figure 1
is a block diagram of the
TFA9891.
In I2S mode, TFA9891 contains three I
2
S data inputs and one I
2
S data output. Two of the
I
2
S inputs, DATAI1 and DATAI2, can be selected as the audio input stream. The third I
2
S
input, DATAI3, is provided to support stereo applications. The selected WS/BCK signal is
used to internally clock DATAI3 (at the output of the MUX controlled by ISEL; see
Figure 1
).
A ‘pass-through’ option allows one of the I
2
S input interfaces to be connected directly to
the I
2
S output. It allows I
2
S output slave device such as codec connected in parallel with
TFA9891 to be routed directly to the audio host via I
2
S output.
In PDM mode, the TFA9891 features one PDM input interface for audio stream and one
PDM output interface to provide AEC voltage or current reference. It also has one I
2
S
input (DATAI3) and one I
2
S output interface (DATAO). In this mode, the I
2
S input/output
interfaces are dedicated to communication between two devices for gain and
synchronization on stereo platforms.
Regardless of active mode (I
2
S or PDM), the I
2
S output signal on DATAO can be
configured to transmit DSP output signal, amplifier output current information, DATAI3 left
or right signal information (bypass) or amplifier gain information. The gain information can
be used to facilitate communication between two devices in stereo applications.
A SpeakerBoost protection algorithm, running on a CoolFlux Digital Signal Processor
(DSP) core maximizes the acoustical output of the speaker. The algorithm also limits
membrane excursion and voice coil temperature to a safe level. The mechanical
protection implemented guarantees that the speaker membrane excursion never exceeds
its rated limit, to accuracy of 10 %. Thermal protection guarantees that the voice coil
temperature never exceeds its rated limit, to accuracy of 10 C. Furthermore, advanced
signal processing ensures that the audio quality remains acceptable always.
The protection algorithm implements an adaptive loudspeaker model that is used to
predict the extent of membrane excursion. The model is continuously updated to ensure
that the protection scheme remains effective even when speaker parameter values
change or the acoustic enclosure is modified.
Output sound pressure levels are boosted within given mechanical, thermal and quality
limits. An optional bandwidth extension mode extends the low frequency response up to a
predefined limit before maximizing the output level. This mode is suitable for listening to
high-quality music in quiet environments.
The frequency response of the TFA9891 can be modified via ten fully programmable
cascaded second-order biquad filters. The first two biquads are processed with 48-bit
double precision; biquads 3 to 8 are processed with 24-bit single precision.
At low battery voltage levels, the gain is automatically reduced to limit battery current. The
SpeakerBoost protection algorithm or the host application (external) controls the output
volume. In the latter case, the boost features of the SpeakerBoost protection algorithm
must be disabled to avoid neutralizing external volume control.
