LX1704
TM
廬
2W Filter less Stereo Class-D Amplifier
P
RODUCTION
D
ATA
S
HEET
FUNCTIONAL DESCRIPTION
WWW .
Microsemi
.C
OM
G
ENERAL
D
ESCRIPTION
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 20dB碌V/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.
F
ILTERLESS
3-L
EVEL
C
LASS
-D M
ODULATION
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
between V
DD
and GND. With zero input voltage, the duty
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
modulation can be described as driving +V
DD
, -V
DD
, and 0V
across the speaker which is why it is referred to as 3-Level
modulation.
Classical, 2-Level modulation drives either +V
DD
or 鈥揤
DD
across the speaker at all times. This scheme requires an L-
C filter between the amplifier鈥檚 outputs and the speaker in
order to keep the output current low.
L
OW
-EMI O
UTPUT
S
TAGE WITH
S
LEW
R
ATE
L
IMITING
AND
A
CTIVE
O
VERSHOOT
C
LAMPING
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.
D
IFFERENTIAL
S
IGNAL
P
ATH
, W
IDE
D
YNAMIC
R
ANGE
,
AND
B
UILT
-
IN
T
HERMAL
O
VERLOAD
P
ROTECTION
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擄C to provide thermal
overload protection.
A
PPLICATIONS
A
PPLICATIONS
Copyright
漏
2005
Rev. 1.2, 2006-10-11
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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