LXE1710 E
VALUATION
B
OARD
I
NDUCTOR
S
ELECTION
The output filter inductors are key elements in the
performance of the Class-D audio power amplifier.
Inductor selection criteria also involves tradeoffs
between performance (efficiency) and component
costs. The critical specifications for the inductor are
the DC resistance, DC current, and peak current
ratings. The inductors should be able to handle the
amplifier’s power as well as operate within its linear
region. Saturating the inductors could decrease
performance (increase THD) and even produce a
short, which may damage either the circuit or the
speaker.
Other variables when selecting an inductor depend on
the switching frequency of the designed amplifier. A
higher switching frequency implies that the corner
frequency of the LC filter is higher. With a higher f
C
,
the inductor value is smaller.
The amplifier’s application and design constraints will
help determine whether the inductors are selected for
size, power, or performance. Various inductors such
as those that are shielded may also have different EMI
effects and distortion performance.
The overall efficiency ( ) of the amplifier circuit is given
in the previous MOSFET section. The inductor’s
power loss contribution is a function of the inductor’s
DC resistance,
R
IND
.
U
SER
G
UIDE
Microsemi
Linfinity Microelectronics Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 12
Copyright
2000
Rev. 1.1, 2000-12-01
)
)(
2
)(
(
Loss
Power
Inductor
2
IND
IND
R
I
P
=
=
The LX1710 Evaluation board utilizes two 15μH radial
leaded R.F. inductors from Inductor Supply, Inc. (ISI).
When evaluating component options, inductors such
as from Coilcraft can be used for other performance /
price tradeoffs. See inductor table below.
P
IND
)(.
2
)(
5
=
The efficiency approximation can now be completed.
P
W
7
)
056
2
=
%
2
90
25
45
.
7
+
56
.
25
+
]
)
(
2
2
2
2
=
+
=
+
+
+
=
+
+
+
+
=
=
L
CROSS
P
IND
DS
L
CROSS
P
L
IND
PDS
R
NDS
R
L
IN
OUT
P
R
I
P
P
R
I
R
R
I
R
I
The efficiency is a function of the power and switching
loss in the MOSFETs and inductors.
Manufacturer
Part Number
Inductance
Q min
Test
Frequency
DC Resistance
DC Current
max (A
RMS
)
Self Resonant
Frequency min
(MHz)
ISI
RL622-150K
15.0
50
2.520MHz
56
2.50
12.0
Coilcraft
DO5022P-153HC
15.0
100kHz
32
4.4
20
Inductor Component Options