LXE1710 E
VALUATION
B
OARD
U
SER
G
UIDE
A
PPLICATION
I
NFORMATION
LX1710 Filter Implementation, 1-stage vs. 2- stage
100
50
20
10
5
2
1
Percent (%)
0.5
0.55626
0.2
0.05
0.02
0.01
0.005
0.002
0.001
50m
100m
200m
500m
1
1.15
Watts(W)
2
5
10
2025.04 30
2-Stage
1-Stage
0.12572
0.1
F
ILTER
D
ESIGN
T
RADEOFFS
(1-
STAGE VS
. 2-
STAGE
)
A 1-stage or 2-stage filter may be used depending on
your application and performance targets. The main
tradeoff in this selection is price (number of
components, component costs, PCB area) vs.
performance. The primary advantage of the single
stage filter is lower cost whereas the main benefit to a
2-stage filter is that it will provide steeper attenuation.
This allows the corner frequency to be selected further
outside of the audio band (to minimize the effects of
impedance variations in the passband) and still
provide adequate RF attenuation.
•
impedance changes could result in a +/- 6dB
change.
THD: There are minimal differences between
the 1-stage and 2-stage implementations with
other parameters such as THD+N as seen in
the above graph.
Single Stage Filter Disadvantages
•
EMI and Switching Frequency: For the 1-
stage, the switching frequency must be higher
than 400kHz to ensure the corner frequency
will provide adequate amplifier performance in
the high end of the audio frequency range. If
f
S
< 400kHz, then
f
C
<
f
S
/10 = 40kHz which is
too close to the desired audio band. A higher
oscillation frequency could translate into
greater MOSFET switching losses, slightly
lower efficiency, and increased EMI effects.
With a 2-stage 4th order filter, the switching
frequency
f
S
can be reduced to 120kHz. If
f
S
=
120kHz, then
f
C
=
f
S
/3 = 40kHz. The lower
oscillation frequency could help minimize EMI
issues.
Single Stage Filter Advantages
•
Low Cost: The 1-stage LC filter uses one half
the number of inductors/capacitors resulting in
a substantial cost savings over a 2-stage
design. Key parameters such as THD+N,
frequency response, and nose performance
do not change significantly.
Power Loss: Since current will flow in two
inductors and not four, the inductor power loss
will be less in the single stage design. The
overall amplifier will have a wider dynamic
range and improved efficiency.
Filter Design: This easy-to-design filter can
limit audio signal changes within +/- 3dB
across the audio band with impedance
vari
" ) * + ,!
a steeper rolloff with the 2-stage filter,
•
•
LC F
ILTER
D
ESIGN
The output filter helps to reconstruct the amplified
audio signal and filter out the switching frequency.
The design of the filter depends on the type of
attenuation and frequency response desired at the
output. The output filter designed into the LXE1710
Copyright
©
2000
Rev. 1.1, 2000-12-01
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 9
MICROSEMI [ MICROSEMI CORPORATION ]
