B: Electrolyticcapacitors

The electrolytic capacitor (C7,C10 in application circuit) parameters to be considered are the following:

a. Value of the capacitance

b. Maximum operating voltage

c. Series inductance/resistance at the switching frequency

c. The electrolytic capacitors supply the ripple current on the output inductance.This ripple is at switching

frequency (around 100Khz). If the inductance/resistance, at the switching frequency, of the capacitor is

high ,there is a strong modulation of the supply lines that can disturb every devices connected to the

same supply lines.

a; b. The application of a Class_D amplifier single eneded ,split supply has the problem of modulation of

the supply in low frequency operation or dc short circuit condition.

Normal power supply are able to deliver current but not to sink it.

We can figure it with an ideal supply with a diode in series (fig. 3).

Figure 3

Figure 4

Let us suppose that the IC is sinking an average current Io from the load , as shown in the previous pic-

ture. The output duty cycle is shown in figure 5.

During the time T1 the current Io is freewheeling throught the capacitor Cup and this leads the +Vcc(IC)

to increase. The freewheling current in Cup continues during all the negative sine cycle of the output.

(the same problem is present in Cdown in positive sine cycle). If the signal frequency is high, the time

duration of the freewheel current is small and so no deep modulation of the +Vcc(IC) is present, but in

low frequency the effect occurs.

It is important to highlight that the total supply on the IC is increased by this freewheel effect. It is impor-

tant, of course, to ensure that the total supply remains under the breakdown limit of the IC (60V).

The maximum supply voltage increase by freewheel is obtained when the output sine wave is around

the half of the possible swing (peak to peak value =Vcc on the avaiable one of 2Vcc). In this condition a

good approximation of the voltage increase can be calculated as

DV

≈

R1

⋅

1

16

⋅

1

C

⋅

2

[3]

The diode highlights the fact that the power supply cannot sink current.

The schematic of the IC in split supply is the following (fig. 4).

AN1013 APPLICATION NOTE

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