LX1704

Microsemi

Analog Mixed Signal Group

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, 800-877-6458, Fax: 714-893-2570

Page 8

2W Filter less Stereo Class-D Amplifier

TM

®

Copyright

© 2009

Rev.1.4, 2009-02-23

FUNCTIONAL DESCRIPTION

The LX1704 is a filterless, low-EMI, class-D audio

power amplifier. It offers high performance (THD+N is

just 0.1% @ 2W), high efficiency (>85% @ 1.2W), and

best in class EMI radiation (just 20dBµV/m). The internal

signal path is completely differential to minimize common-

mode noise pickup. The inputs may be driven single-ended

or differentially and they may be direct or AC coupled. The

LX1704 may be operated with just a single decoupling

capacitor.

The LX1704 output stage is configured as a full H-bridge

push-pull driver. The speaker must be driven differentially

from the OUTP and OUTN pins. Each side of the speaker

is driven by a 200KHz switching signal that transitions

cycle at each output is around 50% and the signals are in-

phase with each other. In this case, there is basically no

differential voltage across the speaker.

When the input

signal goes positive, the duty cycle at OUTP increases

above 50% and the duty cycle at OUTN decreases below

50%. This causes a net positive current to flow into the

speaker. A negative input voltage causes the OUTN duty

cycle to increase and the OUTP duty cycle to decrease

which causes a net negative current to flow into the

speaker. The differential voltage across the speaker has a

fundamental frequency of twice the 200KHz switching

frequency. The speaker itself serves as the low pass filter

which then recreates the audio signal.

This type of

across the speaker which is why it is referred to as 3-Level

modulation.

across the speaker at all times. This scheme requires an L-

C filter between the amplifier’s outputs and the speaker in

order to keep the output current low.

AND

With 3-Level modulation, the carrier frequency drives a

full amplitude common-mode signal to the speaker wires.

This can cause high EMI radiation. One way to combat

this would be to filter the outputs with LC filters or ferrite

beads located close to the amplifier. In the LX1704, the

output stage has been carefully designed to minimize EMI

radiation so that these types of filters are not required.

Slew rate limiting is used to keep the outputs from

switching too quickly. Active overshoot clamping is used

to minimize the inductive overshoot which occurs at each

transition. These two techniques allow the LX1704 to

easily meet FCC standards for radiated emissions when

driving up to 3 meters of speaker wire.

AND

The fully differential signal path uses Pulse Width

Modulation and multiple feedback loops to provide high

performance and low distortion. This is all fully-integrated

to eliminate the need for any external feedback

components or filters.

The gain is fixed by internal

resistors at 20dB. The differential signal path and internal

voltage boosters allow for wide dynamic range. In fact, the

LX1704 can be operated from supplies as low as 2.5V and

as high as 5.5V. The output power will be limited by the

available supply voltage.

An internal thermal sensing

circuit shuts down the outputs when the junction

temperature exceeds about 150°C to provide thermal

overload protection.