- The effects of these spikes are:

Distortion / offset increase due to non linear coupling on internal elementary devices in signal circuit

- Overvoltage on the IC

- Strong noise on logic signal inside the chip, with the possibility of incertain logic levels dangerousfor the IC.

To avoid this spikes generation it is mandatory to put the bypass capacitor at a distance much lower

than 0.5cm from the IC pins. It is important, of course, the quality of the capacitor itself too.

- Other high frequency bypass capacitors that are important for the correct behaviour of the IC are

the following:

C8 (MW15: pin6/8; DIP20: pin 9/1..3,18..20):

Again it is important the low inductance and the nearness to the IC pins.

The voltage on this pin sets the switching frequency of the IC.

The purpose of this capacitor is to filter the high frequency noise that can enter in the IC by pin 6 in

MW15 or pin9 in DIP20. This high frequency noise can generate distortion/offset.

The maximum value of the capacitance that can be used, without switching frequency falls down for high

frequency input signal (max 20Khz), can be calculated with this relation:

1

2

⋅ π ⋅ R4 ⋅ C8

= 40KHz

[1]

This means that C8 can be calculated as:

C8

=

1

2

⋅ π ⋅ R4 ⋅ 40KHz

[2]

For example if R4=12K

Ω [ switching frequency ~ 120Khz] --> C8(max) = 330pF

We suggest C8=280pF.

C4 (MW15:pin5; DIP20 pin 8):

This capacitor sets the bandwidth of the class-D amplifier [see section no.6 ].

It is important that the reference ground of this capacitor is as near as possible to the IC signal ground.

C3 (MW15 pin 9; DIP20 pin 11):

This capacitor filters the high frequency noise that can enter in the input and that can cause intermodula-

tion aliasing noise at the output.

In fact no signal at frequency greater than half of the switching frequency can enter in the IC without

generate aliasing noise

Figure 2

AN1013 APPLICATION NOTE

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