The advantage of this schematic is that it is no longer possible an increase of the overall supply (i.e. in

low frequency operation or in short circuit condition) but only an unbalance between positive and nega-

tive supply (the middle of the devider is no more the half Vcc(tot))

Also in normal operation there can be an unbalance between supply.

DC operation

Due to output offset, there is a dc current that flows in the middle point of the divider GND

Iground = Output offset voltage/load resistance

So the offset of the GND respect to the half Vcc(tot) is:

DV

=

2

For example, with a 50mV offset, Rl = 8Ohm, Rdivider = 1KOhm the offset of GND respect to half Vcc is:

D

=

500

⋅ 50E − 3

8

= 3.1V

It important to note that this GND offset increases as the load decreases (i.e. 4 Ohm)

Low Frequency operation

The impedance of GND is the following:

| Z |

=

1

2

⋅

√

1

For example, with Rdevider = 1KOhm and Cdevider = 2200uF, the GND impedenceat 20 hz is around

1.8Ohm. Because the GND sinks all the current load, this means that ,with 8 Ohm load, on the GND at

20hz there is a modulation of about 1/4 of the output signal.

2. OUTPUT FILTER

To demodulate the pwm signal and obtain the audio signal, it is enought apply a low pass filter at the

output of the amplifier. Of course this filter must not dissipate power. Typical solution is one or more LC

cells in series.

For example, we can consider the case of one only cell at the output (fig 7)

The design of the LC filter must take into account the following parameters:

- Voltage ripple on the load

- Q factor of the LCR filter

- Inductance core : linearity and saturation

- Current ripple in the inductance

Voltage ripple and Q factor

The cut off frequncy of the LC filter is:

=

1

2

L

⋅ C

[4]

For example, with the value suggested in the application circuit, L = 60uH, C = 470nF, the cut-off fre-

quency is Ft = 30Khz.

After this frequency the slope of the Bode diagram is -40 dB/decade.

It is better to fix the cut-off frequency outside the audio bandwidth to avoid the peaking or overdamping

of the LCR filter (the speakers impedance is not purely resistive) inside the audible frequency.

The maximum flat filter is load dependant.

To obtain it , it is important fill the following relation:

1

2

= √

[5]

From [5] it is clear that, fixing the cut-off frequency Ft, for a given load, there is just one value of capaci-

tance C to be used to obtain the maximum flat filter. Then from the [4] we find L.

So, for any load impedance Rl, there is just one maximum flat filter for any cut-off frequency chosen.

From [4] and [5] we can obtain the value of L and C as :

AN1013 APPLICATION NOTE

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