2.3.3 Required input power
The MERUS
TM
audio amp HAT requires a 5 V USB power supply with a minimum of 2.5 A DC current output. It is
designed to be used with the official Raspberry Pi power adapter. MA12070P limiter and volume default
settings have been determined for different nominal loudspeaker impedances while used with the Raspberry Pi
power supply. Details of these settings are covered in section 4.3.
2.3.4 I
2
S digital audio interface configuration
The I
2
S interface is implemented as a slave three-wire communication in the MERUS
TM
audio amp board. As
shown in the schematics, the serial clock line PI_BCLK is also wired to the MA12070P master clock. This
method is used as the Raspberry Pi Zero doesn’t count with a master clock output for this purpose. Supported
sample rates range from 44.1 kHz to 48 kHz and the word length is fixed to 32 bits per channel. The major
applications for wireless audio streaming with the Raspberry Pi are able to decode and/or convert several
audio formats including MP3, AAC, WAV and FLAC to 32 bits, and therefore this should be set in the required
host application, e.g., MPlayer running on Raspbian.
2.3.5 GPIO controls
The MERUS
TM
audio amp board makes use of some of the available GPIOs from the Raspberry Pi for safe boot-
up and shut-down sequences. These are automatically handled by the software drivers. During boot-up, at first
the boost converter and MA12070P are disabled. Next, the boost converter will be enabled until the 20 V
bypass capacitors are fully loaded. Finally, the MA12070P will be enabled. With regard to the mute control pin,
the amplifier will be always muted until PCM streaming is present. This will avoid pop-noises during non-
playback time and also during boot-up and shut-down sequences.
2.3.6 Short-circuit protection
The MA12070P amplifier has built-in protection mechanisms such as undervoltage lockout (UVLO),
overtemperature warning/error, short-circuit/over-load protection, power stage pin-to-pin short-circuit, and
DC protection. Additionally, the MERUS
TM
audio amp board counts with external protection diodes D1 to D4 at
the output-in Figure 4 to avoid inrush currents under any circumstances.
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