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TPA3136D2, TPA3136AD2

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SLOS938F – MAY 2016 – REVISED JANUARY 2020

Product Folder Links: TPA3136D2 TPA3136AD2

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Copyright © 2016–2020, Texas Instruments Incorporated

Typical Applications (continued)

10.2.2 Detailed Design Procedure

A rising-edge transition on SD input allows the device to start switching. It is recommended to ramp the PVCC

voltage to its desired value before releasing SD for minimum audible artifacts.

The device is non-inverting the audio signal from input to output.

The GVDD pin is not recommended to be used as a voltage source for external circuitry.

10.2.2.1 Ferrite Bead Filter Considerations

Using the Advanced Emissions Suppression Technology in the TPA3136D2, TPA3136AD2 amplifier it is possible

to design a high efficiency Class-D audio amplifier while minimizing interference to surrounding circuits. It is also

possible to accomplish this with only a low-cost ferrite bead filter. In this case it is necessary to carefully select

the ferrite bead used in the filter.

One important aspect of the ferrite bead selection is the type of material used in the ferrite bead. Not all ferrite

material is alike, so it is important to select a material that is effective in the 10 to 100 MHz range which is key to

the operation of the Class-D amplifier. Many of the specifications regulating consumer electronics have

emissions limits as low as 30 MHz. It is important to use the ferrite bead filter to block radiation in the 30-MHz

and above range from appearing on the speaker wires and the power supply lines which are good antennas for

these signals. The impedance of the ferrite bead can be used along with a small capacitor with a value in the

range of 1000 pF to reduce the frequency spectrum of the signal to an acceptable level. For best performance,

the resonant frequency of the ferrite bead/ capacitor filter should be less than 10 MHz.

Also, it is important that the ferrite bead is large enough to maintain its impedance at the peak currents expected

for the amplifier. Some ferrite bead manufacturers specify the bead impedance at a variety of current levels. In

this case it is possible to make sure the ferrite bead maintains an adequate amount of impedance at the peak

current the amplifier will see. If these specifications are not available, it is also possible to estimate the bead's

current handling capability by measuring the resonant frequency of the filter output at low power and at maximum

power. A change of resonant frequency of less than fifty percent under this condition is desirable. Examples of

ferrite beads which have been tested and work well with the TPA3136D2, TPA3136AD2 device include

NFZ2MSM series from Murata.

A high quality ceramic capacitor is also needed for the ferrite bead filter. A low ESR capacitor with good

temperature and voltage characteristics will work best.

Additional EMC improvements may be obtained by adding snubber networks from each of the class-D outputs to

ground. Suggested values for a simple RC series snubber network would be 68

Ω in series with a 100-pF

capacitor although design of the snubber network is specific to every application and must be designed taking

into account the parasitic reactance of the printed circuit board as well as the audio amp. Take care to evaluate

the stress on the component in the snubber network especially if the amp is running at high PVCC. Also, make

sure the layout of the snubber network is tight and returns directly to the GND or the thermal pad beneath the

chip.

10.2.2.2 Efficiency: LC Filter Required with the Traditional Class-D Modulation Scheme

The main reason that the traditional class-D amplifier needs an output filter is that the switching waveform results

in maximum current flow. This causes more loss in the load, which causes lower efficiency. The ripple current is

large for the traditional modulation scheme, because the ripple current is proportional to voltage multiplied by the

time at that voltage. The differential voltage swing is 2 × VCC, and the time at each voltage is half the period for

the traditional modulation scheme. An ideal LC filter is needed to store the ripple current from each half cycle for

the next half cycle, while any resistance causes power dissipation. The speaker is both resistive and reactive,

whereas an LC filter is almost purely reactive.

The TPA3136D2, TPA3136AD2 modulation scheme has little loss in the load without a filter because the pulses

are short and the change in voltage is VCC instead of 2 × VCC. As the output power increases, the pulses widen,

making the ripple current larger. Ripple current could be filtered with an LC filter for increased efficiency, but for

most applications the filter is not needed.

An LC filter with a cutoff frequency less than the class-D switching frequency allows the switching current to flow

through the filter instead of the load. The filter has less resistance but higher impedance at the switching

frequency than the speaker, which results in less power dissipation, therefore increasing efficiency.