Tai-1 Microelectronics

TMPA401DM

www.taimec.com.tw

May 25, 2005

Copyright

©2004,Tai-1 Microelectronics Corp.

8

In this example Rext=22k ohms, Zin=20k ohms and C=C2=0.47nF. Thus the

–3db frequency

at the high frequency side is about 16kHz.

Input filter

Since the input pins of the device have high impedance it is suggested that an input filter is

incorporated as shown in the diagram. The AC coupling capacitors are used to block the DC

voltage from the device. They also define the

–3db frequency at the low frequency side. A

bypass capacitor placed in between the positive signal path and negative signal path is to

attenuate the high frequencies. It defines the

–3db frequency at the high frequency side. The

input filter becomes a band pass filter.

The

–3db frequency of the low frequency side is

f-3db = 1/ 2πR C

where C is the AC coupling capacitance(1uF) and R is the total resistance in series with C.

The

–3db frequency of the high frequency side is

f-3db = 1/ 2πR C

where C is the bypass capacitance(1nF) and R is the total resistance in parallel with C.

Note that there is 20K input resistor integrated in the chip for each input INP/INN.

Output filter

Ferrite bead filter can be used for EMI purpose. The ferrite filter reduces EMI around 1 MHz

and higher（FCC and CE only test radiated emissions greater than 30 MHz）. When selecting a

ferrite bead, choose one with high impedance at high frequencies, but low impedance at low

frequencies.

Use an LC output filter if there are low frequency（＜1 MHz）EMI sensitive circuits and/or there

are long wires from the amplifier to the speaker. EMI is also affected by PCB layout and the

placement of the surrounding components.

The suggested LC values for different speaker impendence are showed in following figures for

reference.

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