AN1013 데이터 시트보기 (PDF) - STMicroelectronics

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AN1013

MONO CLASS_D AMPLIFIERS

STMicroelectronics 

AN1013 APPLICATION NOTE

Figure 8

Figure 9

C

=

2

⋅

π

⋅

1

Ft ⋅ R1

⋅

√

2

[6]

L

=

4

⋅

π2

1

⋅ Ft2

⋅

C

[7]

For example, for Rl=8Ohm and Ft=30Khz, we can obtain, using the [6] [7] , the following values of L and C:

[6] --> C = 469nF; [7] --> L = 60µH

If the switching frequency is set at 120Khz the voltage ripple at the switching frequency on the 8 Ohm

load, at ±20V, with L=60µH, C=470nF, is Vripple ~ 2V (the amplitude of the ripple is proportional to the

Vcc). The ripple is not audible (~100Khz) and usually is not a problem. If it is, the solution is to add one

or more LC filter cells at the output of the IC.

Inductance core

The core permeability has: hysteresis, not linearity, saturation. The hysteresis produces losses in the

core. These losses affect the overall system efficiency but not, at first order, the dissipated power of the

IC. A high losses core generally produces an increase of the bias current. The non linearity of the per-

meability with the increase of the magnetic field deteriorates the THD, usually the 3^ order harmonic, but

not the sound quality because it is a low order harmonic.

Saturation of core can be dangerous because, for high current, it can transform the inductance in a

short-circuit with all the risks related to this situation.

Current ripple

As first approssimation the peak value of the current in the inductance is (in no signal condition):

Iripple

=

VCC

L

⋅

T

2

⋅

1

2

[8]

The decrease of the inductance value generates an increase of the ripple current in the inductance. For

example with a switching frequency of 100Khz and +/-20V supply, the peak current ripple are the following:

= 60µH; Iripple = 0.83A

= 30µH; Iripple = 1.66A

(i.e. L=60µH is the inductance of the maximum flat filter for 8Ω load with a cut off frequency of 30Khz

(C=470nF); L=30µH is the inductance of max flat filter for 4Ω load with cut off frequency of 30Khz

(C=940nF). It is important to note that the ripple current must stay under the threshold of the short circuit

protection (see data-sheet). The increase of the inductance over the maximum flat filter value, generates

an overdumped filter frequency responce (see fig. 8) and an anticipated dead zone step (see section 5)

.

3. SNUBBER

The snubber must be chosen to dump the overshoots produced at each commutation. To reach this

goal, it is important that the snubber impedence seen by the circuit at frequency of the overshoot is re-

sistive. The smaller is the resistance, the higher is the dumping produced by the snubber and so the

smaller are the overshoots (but the losses on the snubber increases).

6/13

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