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Maxim > Design
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> Audio
Circuits > APP 4526

Aug 17, 2010

Keywords: class D audio amplifiers, switchmode
boost converters, AA alkaline cells
Related
parts

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APPLICATION NOTE 4526

Two AA Cells Produce Huge Audio Volume

Abstract: Class
D audio amplifiers like the MAX9704 offer almost 90%
efficiency, but they need 10V to operate. By incorporating a
switched-mode boost converter (MAX1771), which generates 12V
from inputs down to 2V and delivers output currents up to two
amperes, this class D amplifier can operate from a pair of AA
cells.

Attend
this brief webcast by Maxim on
TechOnline

A
similar version of this article appeared in the March 11, 2008
issue of Electronic Design magazine.

Class D
audio amplifiers from Maxim offer almost 90% efficiency, but
they need 10V to operate, and (for battery-operated systems)
10V requires a large number of batteries wired in series. You
can avoid bulky battery packs by incorporating a switched-mode
boost
converter (MAX1771), which
generates 12V from inputs down to 2V, and delivers output
currents up to two amperes. The battery-operated class D
amplifier can then operate from a pair of AA
cells.

Maxim provides evaluation boards (EV kits) for
both the Class D amplifier (MAX9704) and the
boost converter. You simply wire them together and connect two
AA alkaline cells, an MP3 player for the audio source,
and a pair (in this case) of Celestion Ditton 44 loudspeakers
(Figure 1).

More
detailed image (PDF, 39MB)
Figure 1. Powered by a
switching converter operating from two AA cells, this Class D
amplifier (right-hand board) delivers 6W of speaker power for
as long as four hours.

The results are stunning.
Power output is limited by the MAX1771EVKIT,
but the total available power, estimated at 6 watts, is
utilized with high efficiency by the class D amplifier. The
volume from this simple circuit is amazing, and allows the
estimated battery life of two AA cells (over 4 hours) to
provide a full evening of loud entertainment.

The
technique can also provide uninterrupted audio in an
automobile, allowing entertainment to continue at full volume
while the engine is being cranked. That capability will be of
increasing interest when a proposed law requires car engines
to stop at traffic lights, and then re-start when the throttle
is depressed. The voltage
from a 12V battery dips as low as 6V when the car is cranked
during cold conditions, and that drop in supply voltage can
have a substantial effect on the audio amplifier. Rear-seat
applications can also benefit from this circuit, because DVD
players initiate a relatively long reset period when their
power supply is interrupted. DVD-based navigation systems,
too, need protection from supply dropouts.

Again, the
main limitation on power output is the MAX1771EVKIT, whose
maximum output current
at 6V is 0.5A. The kit can be modified to deliver as much as
2A, which boosts the power output (at 6V) to 24W. Two AA
batteries can support this power output as before, but perhaps
not for four hours!

Related parts

MAX1771

12V or Adjustable,
High-Efficiency, Low IQ, Step-Up DC-DC
Controller

Free
samples

MAX1771EVKIT

Evaluation Kit for the
MAX1771

MAX9704

10W Stereo/15W Mono,
Filterless, Spread-Spectrum, Class D Amplifiers

Free
samples

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Download,
PDF format (88kB)
APP 4526: Aug 17, 2010

AN4526, AN 4526, APP4526,
Appnote4526, Appnote 4526

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