Figure 10

If Tosc is the period of the overshoot, the snubber must fit this relation (impedance at overshoot fre-

quency: resistive):

1

2

⋅

<

1

[9]

The power dissipated by the resistance of the snubber, on average, can be approximated as:

=

2

R

⋅

DT

T

[10]

where DT is the commutation time of the output.

For example, let us assume Vcc = 20V (+-20V),Tosc =300ns, R = 150

Ω, Dt=50ns, T = 8.3µs (Freq.

switching = 120Khz). In this case the capacitance to be used is (from [9] ) C >318pF (i.e. in the applica-

tion circuit: C = 560pF).

The power dissipated by the resistance of the snubber is (from [10]) Psnub ~ 16mW.

This is a very ’soft’ snubber.

The strongest snubber that can be used to cut overshoot at frequency higher than about 2Mhz with

1/4W resistance , switching frequency 120 Khz and Vcc = 20V (+-20V), is:

R= 10

Ω (1/4 W), C > 8nF (i.e. 12nF)

5. THD: SWITCHING FREQUENCY CHOICE

The THD of a class_D amplifier is due mainly to :

- Dead zone to avoid cross-conduction in the power stage

- Parasitic coupling of spikes in the signal circuit

- Non linearity of the modulation process

- Aliasing of the PWM spectrum inside the audio bandwidth

- The aliasing of the pwm spectrum inside the audio bandwidth is present for high frequency signal ap-

plied to the IC. For a switching frequency of 120 Khz no sensible aliasing was measured for input sig-

nal up to 20 Khz.

It seems enough to ensure good performance for audio signal. The eventual increase of the switching

frequency improve this figure but worsens the efficiency and the dead zone distortion and makes the op-

eration of the power stage more difficult.

We suggest a switching frequency between 110 -150 Khz.

The resistance R4 (pin 9 DIP20, pin 6MW15) set the switching frequency of the internal multivibrator.

The relation is :

=

Bf

R4

(Bf =1.4E9 Hz Ohm)

[11]

AN1013 APPLICATION NOTE

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