TAS5721 datasheet(55/71 Pages) TI1

55 / 71 page

TAS5721

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SLOS739 – JULY 2012

Supply Pumping and Polarity Inversion

The high degree of correlation between the left and right channels of a stereo audio signal dictates that, when

the left audio signal is positive, the right audio signal tends to be positive as well. When the Class D is configured

for single-ended operation (as would be the case for Single Device 2.1 Operation), this results in both outputs

drawing current from the supply rail "in phase". Similarly, when the left audio signal is negative, the right audio

signal tends to be negative as well. For single-ended operation, both outputs will likewise force current into the

ground rail. This can lead to a phenomenon called "supply pumping" in which the capacitances on the PVDD rail

begin to store charge- raising the voltage level of PVDD as well. This noise injection onto the rail is in phase with

and at a similar frequency of the signal being produced by the amplifier output stage. This phenomenon can

cause issues for other devices attached to the PVDD rail. The problem does not occur for BTL outputs since

outputs of both polarities are always present for each channel.

To combat supply pumping in 2.1 Mode, the device has an integrated speaker-mode volume negation feature,

which, essentially introduces a polarity inversion (shift by 180°) to any of the given channels. By setting the

correct bit in 0x20[31:24], it is possible to invert the polarity of the DAP channels that drive the PWM modulator

blocks. This allows, for instance, the left channel to operate with its default polarity, while the right channel could

have its polarity inverted to balance current flow into and out of the supplies. This procedure could have an

adverse implication on the stereo imaging of the audio system because, if the speakers in the system are

connected in the same manner as they would be connected when being driven by traditional BTL channels, the

phase of the signals being sent to the speakers is 180° out of phase. In order to prevent this from occurring, the

speaker on the negated channel must be connected "backwards" (i.e. the Class D signal for the negated channel

gets connected to the negative speaker terminal and the positive terminal is grounded). In this way, supply

pumping is reduced while keeping the effective signal polarity the same. The table above includes register

settings which enable the polarity inversion, so care should be taken to adjust the polarity of the speakers if this

feature is left enabled. Of course this feature can be left disabled if desired, provided the supply pumping

phenomenon doesn't cause any other system level issues

PBTL-Mode Support

The TAS5721 supports parallel BTL (PBTL) mode with OUT_A/OUT_B (and OUT_C/OUT_D) connected after

the LC filter. In order to put the part in PBTL configuration, the PWM output multiplexers should be updated to

set the device in PBTL mode. Output Mux Register (0x25) should be written with a value of 0x01 10 32 45. Also,

the PWM shutdown register (0x19) should be written with a value of 0x3A.

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