The well-known LM386 is
an excellent choice for many designs requiring a small audio power
amplifier (1-watt) in a single chip. However, the LM386 requires quite a
few external parts including some electrolytic capacitors, which
unfortunately add volume and cost to the circuit. National Semiconductor
recently introduced its Boomer® audio integrated circuits which were
designed specifically to provide high quality audio while requiring a
minimum amount of external components (in surface mount packaging only).
The LM4906 is capable of delivering 1 watt of continuous average power
to an 8-ohm load with less than 1% distortion (THD+N) from a +5 V power supply. The chip happily works with an external PSRR (Power Supply Rejection Ratio) bypass capacitor of just 1 µF minimum.
In addition, no output coupling capacitors or bootstrap capacitors
are required which makes the LM4906 ideally suited for cellphone and
other low voltage portable applications. The LM4906 features a low-power
consumption shutdown mode (the part is enabled by pulling the SD pin
high). Additionally, an internal thermal shutdown protection mechanism
is provided. The LM4906 also has an internal selectable gain of either 6
dB or 12 dB. A bridge amplifier design has a few distinct advantages
over the single-ended configuration, as it provides differential drive
to the load, thus doubling output swing for a specified supply voltage.
Four times the output power is possible as compared to a single-ended
amplifier under the same conditions (particularly when considering the
low supply voltage of 5 to 6 volts).
When pushed for output power, the small SMD
case has to be assisted in keeping a cool head. By adding copper foil,
the thermal resistance of the application can be reduced from the free
air value, resulting in higher PDMAX values
without thermal shutdown protection circuitry being activated.
Additional copper foil can be added to any of the leads connected to the
LM4906. It is especially effective when connected to VDD, GND,
and the output pins. A bridge configuration, such as the one used in
LM4906, also creates a second advantage over single-ended amplifiers.
Since the differential outputs, Vo1 and Vo2, are biased at half-supply,
no net DC voltage exists across the load.
This eliminates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configuration. Large
input capacitors are both expensive and space hungry for portable
designs. Clearly, a certain sized capacitor is needed to couple in low
frequencies without severe attenuation. But in many cases the speakers
used in portable systems, whether internal or external, have little
ability to reproduce signals below 100 Hz to 150 Hz. Thus, using a large
input capacitor may not increase actual system performance. Also, by
minimizing the capacitor size based on necessary low frequency response,
turn-on pops can be minimized.