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Pump up the volume! Enhance audio quality and power in mobile phones and portable media players

Intro

When it comes to delivering high-power and high-quality audio in portable devices, some inherent problems exist such as weak bass, distortion, flat-sounding music, etc. This article discusses a power architecture (BriteSound) that uses a thin, prismatic supercapacitor combined with a Class-D audio amplifier with an integrated boost converter to deliver five-watt power bursts to offload peak-power functions from the battery. The integrated boost converter effectively manages supercapacitor inrush current at power on. This combination of components ” a prismatic supercapacitor and a Class-D audio amplifier with an integrated boost converter – increases peak audio power while saving the designer both board space and component cost. It can also multi-task to enable high-power LED flash photography without compromising a mobile handset's thin profile.

This paper builds on a previously published article that also describes a BriteSound power architecture solution, but in this case a prismatic supercapacitor powered a generic Class-D audio amplifier, Supercapacitors enhance audio quality and power in mobile phones. [1]

Audio quality problems in portable audio devices

Consumers are demanding ever more high performance from battery-powered
portable audio devices. Mobile phones, portable media players (PMP) and
personal navigations devices are typical applications where the audio
output needs to be loud, clear and high quality. These devices, powered
by a Lithium-Ion (Li-Ion) battery, typically power a Class-D audio
amplifier directly across the battery, usually at ~3.7V. Figure 1 shows
this typical case which limits peak power for an 8 ohm speaker to
3.7V2 /8 ohm = 1.7W, or 3.4W for a stereo pair. The battery current
for peak stereo audio power = 3.4W/3.7V = 0.92A. This arrangement
results in an audio playback capability which can suffer from power
limitations, distortion and interference.


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on image to enlarge.

Figure 1: Typical configuration for Class-D amplifier

An alternative arrangement is for a PMP to use one or more AA
or AAA batteries in series to power a boost or buck-boost converter,
which supplies the audio amp at 5V as shown in Figure 2.


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on image to enlarge.

Figure 2: Class-D amplifier with boost converter

In this case, peak power is limited by the impedance of the
alkaline batteries and the current they can deliver, as well as by the
peak inductor current the boost converter can handle. As an example, if
a PMP is powered by two AA batteries, then the internal impedance of
these two batteries in series is approximately 500m ohm. Therefore, if
the PMP attempts to deliver 3.4W of power as shown in Figure 2, the
unloaded battery voltage equals 3V. Assuming a boost converter
efficiency of 90 percent, the battery current can be calculated as:


Click
on image to enlarge.

where Vbatt(unloaded) = 3V, LoadPower = 3.4W, efficiency =
90%, Rbatt = 0.5 ohm.

The battery current equals 1.8A. With 30% ripple, the peak
inductor current will be 2.1A. However, this is not practical to
implement. The inductor is too large, and battery voltage will drop to
2.1V.

Powering an audio amplifier through a boost converter
with a supercapacitor at its output


An alternative solution to both these architectures that enables higher
peak power with reduced peak battery current, and is still thin enough
for a slim mobile handset, is to power the audio amplifier through a
boost converter with a prismatic supercapacitor such as a CAP-XX HW209
(140mF, 120m ohm), HW202 (220mF, 90m ohm), HW203 (550mF, 90m ohm) or
HA230 (425mF, 110m ohm) at its output as shown in Figure 3. We chose
the HW203 for our tests.


Click
on image to enlarge.

Figure 3: Class-D amplifier with boost converter and
supercapacitor

Powering the audio amp from a boost converter with output
voltage at 5.5V increases peak power for a pair of 8 ohm speakers to 2
x 5.5V2 /8 ohm = 7.6W, or approximately double the 3.7W for the two
cases considered above. However, using a boost converter without a
supercapacitor draws too much current from the battery. As an example,
consider a boost converter that is 85% efficient and powered from a
Li-Ion battery at 3.7V, with battery pack plus a connector impedance of
150m ohm. Then from Equation 1, the peak battery current at peak audio
power would be 2.7A, which is too great to be practical for a mobile
phone. The supercapacitor enables battery and boost converter to supply
average audio current, while the supercapacitor, with a low ESR (~100m
ohm), supplies peak current. If the music has a crest factor of 6dB,
then the boost would supply an average 2.2W, so from eqn (1) the
battery would only deliver 0.67A which it can easily do.

The boost converter needs to be specifically designed to limit
the inrush current to charge the supercapacitor from 0V. This is
because a boost has a DC current path when Vout < Vin. Consider Figure 4.


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on image to enlarge.

Figure 4: Standard boost converter topology cannot
limit supercapacitor inrush current

When Vout > Vin-VD , the NFET is off and there is a DC
current path through L1 & D1. If Vin = 3.7V, VD is 0.3V, DC
resistance of L1 = 50m ohm, the source impedance supplying Vin is 100m
ohm, supercapacitor ESR = 100m ohm and C supercapacitor
>> C2, then inrush current


Click
on image to enlarge.

This clearly shows that a standard boost converter cannot
manage the inrush current when first connected to a supercapacitor. In
order to limit inrush current, D1 must be replaced by a PFET where the
body diode can be biased out, so the PFET can be operated in linear
mode to limit forward current when Vout < Vin.

Using an audio amplifier with an integrated boost
converter


The TPA2013D1
includes a Class-D audio amplifier with an integrated boost converter
with an input voltage range of 1.8V – 5.5V, and an output of 5.5V.
Figure 6 shows a music waveform driving an 8 ohm speaker with the
topology as shown in Figure 5, where Vin = 3.6V supplied from a Li-Ion
battery. Peak speaker voltage is 5.4V, which corresponds to 3.8W audio
power. At that peak power, input current drawn from the battery equals
1.1A. A stereo pair of speakers with similar topology would draw over
2A, which places too great a load on a mobile phone battery.


Click
on image to enlarge.

Figure 5: Topology using the TPA2013D1 without a
supercapacitor


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on image to enlarge.

Figure 6: Music waveform showing speaker voltage and
battery current for topology shown in Figure 5

The TPA2013D1 has short circuit protection an all outputs.
This protects the output of the integrated boost from supercapacitor
inrush current and enables a supercapacitor to be used with the
TPA2013D1. This is achieved by biasing out the body diode of the output
PFET of the boost, and driving it in linear mode to control current
when Vout < Vin. Figure 7 shows the supercapacitor charge current and voltage when charged from 0V.


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on image to enlarge.

Figure 7: Supercapacitor charging is well controlled
by the TPA2013D1

Figure 9 shows the same piece of music as Figure 6, with a
supercapacitor connected to the output of the boost converter, as shown
in Figure 8.


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on image to enlarge.

Figure 8: Supercapacitor at output of b oost
converter supplies peak currents to audio amp


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on image to enlarge.

Figure 9: The same piece of music as in Figure 6, but
with peak audio current supplied by the supercapacitor


Click
on image to enlarge.

Key Points

– Peak audio power in Figures 6 and 9 is the same at 3.7W. However,
with a supercapacitor supporting the peak load, peak battery current
has been halved from 1.11A to 0.54A.

– With a supercapacitor, the same boost converter could support a pair
of audio amplifiers for stereo sound. For example, a TP2032D1, which
has 2V/V gain, could be powered from the TPA2013D1 boost with
supercapacitor for stereo, with the gain setting for the TPA2013D1 at
2V/V (Gain pin tied to 0V).

– Without the boost converter integrated into the TPA2013D1, powering
the audio amp directly from the battery, the peak power would be only
~1.5W. This assumes the battery voltage is 3.7V, the speaker impedance
is 8 ohm and the amplifier is 90 percent efficient. The integrated
boost converter in the TPA2013D1 doubles the peak audio power.

– The block diagram in Figure 8 shows a pair of 10K ohm balancing
resistors to maintain equal voltage across the two supercapacitor
cells. This draws ~275 micro amps, which may be too high for some
applications. An alternative is to use a low-power operational
amplifier (op amp) for an active balance circuit which will draw a
total of 2 to 3micro amps.

Conclusions

From the results presented in this article, we can see that
the integrated boost converter more than doubles peak audio power when
compared to driving the audio amplifier directly from a Li-Ion battery.
Without a supercapacitor, this configuration more than doubles the peak
battery current. An integrated boost converter is suitable with a
supercapacitor connected at Vout of the boost converter. It manages the
inrush current that a supercapacitor draws upon power up. The
supercapacitor halves the peak battery current when used with the
TPA2013D1, enabling peak audio power to be doubled by supplying the
audio amp at 5.5V without increasing the strain on the battery,
compared to the case of supplying the audio amp directly from the
battery.

The increased audio power and stiffer audio amplifier supply
rail made possible by combining the Texas Instruments TPA2013D1 with a
prismatic CAP-XX HW203 supercapacitor makes the music sound much fuller
and richer.

References

– [1] Mars, Pierre, “Supercapacitors enhance audio quality
and power in mobile phones,” AudioDesignLine.com, June 5, 2007: http://www.audiodesignline.com/199901518.

– [2] To download a datasheet and other technical documents or to order
samples for the TPA2013D1, visit: http://www.ti.com/tpa2013d1-ca

.
– [3] The charts and downloadable product list available from http://www.cap-xx.com/products/products.htm
provide technical details on CAP-XX supercapacitors. Sample quantities
are available via the “Buy Now” button.

About the Authors

Pierre Mars is the VP of Applications Engineering for Sydney,
Australia-based CAP-XX Ltd. He can be
reached at .

Sachin Ranganathan is the Portable Audio Marketing Manager and Don
Dapkus is an Applications Engineering Manager for the Audio Power
Amplifier group at Texas Instruments. Don and Sachin can be reached at .

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