Microsemi

Linfinity Microelectronics Division

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

Page 10

Copyright

© 2000

Rev. 1.1, 2000-12-01

evaluation board is a second order, LC type filter as

shown below.

Tradeoffs between performance and

component cost must be considered when determining

the complexity or type of filter selected.

OUT+

OUT-

L

15µH

L

15µH

C

0.68µF

C

0.68µF

R

Its Laplace Transform function is:

ω

ω

ω

ω

RC

Q

LC

S

Q

S

C

S

LC

S

RC

S

C

S

=

=

+

+

=

+

+

=

1

Where

1

1

H(S)

2

2

2

The Class-D amplifier evaluation board design has a

pass-band of 20Hz to 20kHz to support the audio

frequency range and is configured to utilize a switching

application, this oscillator frequency may be adjusted

(see section on Oscillator Configuration) to optimize

amplifier

performance

or

modified

for

other

considerations

such

as

EMI

effects.

Further

requirements of the filter are that the pass band

than 40dB and the corner frequency of the LC filter

should be set higher than 20kHz to avoid attenuating

audio signals in the desired audio band by more than

1dB.

A speaker DC impedance o

50kHz corner frequency are defined for the evaluation

board.

The Q (selectivity factor or ratio of the center

frequency divided by the bandwidth) of the filter must

also be considered when designing a filter. Too high a

Q will result in a boost of the audio signal across the

audio band whereas a low Q will cause too much

attenuation of the signal. A Q value of 0.707 provides

the required audio response and is used in the

calculation below.

Board

Evaluation

in the

used

is

µF

68

.

0

µ

56

.

0

)

50000

)(

2

(

4

707

.

0

)

f

2

(

=

=

=

=

=

C

F

R

Q

R

Q

C

C

π

π

ω

To Compute the Inductor Value:

Board

Evaluation

in the

used

is

µH

15

µ

9

.

14

)

µ

68

(.

)]

50000

)(

2

[(

1

)

f

2

(

1

1

2

2

2

=

=

=

=

=

L

H

C

C

L

C

π

π

ω

LXE1710 Evaluation Board

Frequency Response

-15

+15

-12.5

-10

-7.5

-5

-2.5

+0

+2.5

+5

+7.5

+10

+12.5

10

80

k

20

50

10

20

50

1k

2k

5k

10

k

20

k

50

k

Frequency (Hz)







Frequency response of the audio amplifier was

 $    " )

  ,!

- $-  + - - 

"" ' .  
 ! - ,

 ) " +  )/ .   –4dB

attenuation

respectively.

Therefore,

to

improve

frequency response performance for other loads, the

value of Q must be increased/decreased by changing

the capacitor. Since a different value C will affect the

corner frequency, values for L and C must be

recalculated.

Below are recommended inductor and

capacitor values for 2


  ,   -

single stage LC filter design.

Capacitor C (µF)

Inductor L (µH)



1.0

10



0.68

15



0.47

22

  Filter Component Values

for

speaker

loads.

Actual

speakers

have

varying

impedances, which may require revised filter calculations

and optimization. Furthermore, your application may have

different design goals than those chosen for the LX1710