NXP Semiconductors
TFA9872_SDS
High Efficiency Class-D Audio Amplifier with Speaker-as-Microphone
TFA9872_SDS All information provided in this document is subject to legal disclaimers. © NXP B.V. 2017. All rights reserved.
Product short data sheet Rev. 1 — 12 April 2017
COMPANY PUBLIC 9 / 32
8 Functional description
The TFA9872 is a highly efficient Bridge Tied Load (BTL) class-D audio amplifier
embedding Speaker-as-Microphone (SaM) support, as depicted in the block diagram of
Figure 1.
The TFA9872 contains a TDM/I
2
S input/output interface for communicating with the audio
host. The maximum number of slots is 16 (at f
s
= 48 kHz) and the minimum number is 2
(I
2
S mode). The interface is compliant with all I
2
S interface configurations and supports a
wide range of TDM interface configurations. It also features an optional ultrasonic path to
the speaker.
The TFA9872 features a slave-configurable IN or OUT PDM interface. This audio
interface can be connected to the audio host to provide a low-latency path (for side
tone mixing) to the speaker. The 1-bit PDM stream is decimated and applied to the
TDM interface. The PDM stream can, optionally, be attenuated or amplified by the gain
module. Soft mute control prevents pop and click noise occurring when this signal path is
switched on or off. The PDM output also provides a SaM stream back to the host.
At low battery voltage levels, the gain is automatically reduced to limit battery current
(when battery safeguard is enabled).
The digital audio stream is converted into two PWM signals which are then injected into
the class-D audio amplifier. The 3-level PWM scheme supports filterless speaker drive.
An adaptive DC-to-DC converter boosts the battery supply voltage when the audio
stream crosses two programmable voltage thresholds. It switches automatically to
Follower mode (V
BST
= V
BAT
; no boost) when the audio output voltage is lower than the
battery voltage.
The SaM feature is available in both PDM and I
2
S modes. This function can be used to
turn the speaker into a dynamic microphone, providing an output audio stream on the
digital interfaces. Due to the nature of the speaker membrane, the microphone equivalent
characteristics perform best in high Sound Pressure Level (SPL) environments.
Consequently, this feature is targeted at specific use cases such as concert recording or
calls affected by wind noise. It is not intended to replace a primary/standard microphone
but rather to complement it in such use cases by providing a signal that is less sensitive
to saturation.
For SaM, a dedicated PGA is used to amplify the weak signal coming from the main
speaker or receiver speaker. The result is a microphone that can handle high SPL
environments.
SaM can be enabled on the main speaker connected to OUTA/OUTB, when the amplifier
is off. Alternatively, SaM can be enabled on auxiliary inputs AUXSAMP/AUXSAMN, with
the receiver speaker connected as input.
