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PDF AN1013 Data sheet ( Hoja de datos )
| Número de pieza | AN1013 | |
| Descripción | MONO CLASS_D AMPLIFIERS | |
| Fabricantes | ST Microelectronics | |
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AN1013
APPLICATION NOTE
INTRODUCTION
MONO CLASS_D AMPLIFIERS
by M. Masini, L. Pagotto
The TDA7480/81/82 are single ended , split supply, class_D amplifiers. The output of the amplifier is a
high frequency square wave (around 100Khz), rail to rail, with variable duty cycle.
The audio information is the average value of the output square wave.
To obtain the audio signal, the output must be low pass filtered.
The main issue of this amplifier is the very low dissipated power (the very high efficiency) compared to a
normal class AB amplifier.
The preamplifier provides the voltage gain of the overall amplifier. The second stage is the power stage,
with a gain 1.5, that is the high efficiency class_D amplifier.
The class_D amplifier stage is done with a multivibrator : with no signal it generates a 50% duty cycle
square wave, with signal applied, it changes the duty cycle.
The switching frequency is set by the voltage on pin 9 (DIP20) or pin 6 (MW15).
The output power stage is done with N-ch DMOS power with the upper one supplied by a bootstrap ca-
pacitor (C11 in the application circuit).
1. CRITICAL COMPONENTS IN THE APPLICATION
A: Bypass high frequency filtering capacitor on supply
The most important filter capacitor is C5 (see application circuit) between pin 13/14 for MW15 package
or pin 16/17 for DIP20 package.
The value of the parasitic inductance between this capacitance and the IC pins is related to the ampli-
tude of the spikes on the power supply pins at every commutations of the output.
In fact, for any commutation, there is an abrupt variation of the current in the parasitic inductances Lpar
in series to the supply. This abrupt variation increases as the output current increases and can be typi-
cally of some amperes on 10ns. With this slew rate of the current, also an inductance of about some the
of nH (i.e. the lead inductance of the pin) generates voltage spikes of some volts.
Figure 1. Block Diagram
February 1998
1/13
1 page  
AN1013 APPLICATION NOTE
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
=
Rdivider
2
⋅
Iground
For example, with a 50mV offset, Rl = 8Ohm, Rdivider = 1KOhm the offset of GND respect to half Vcc is:
D
=
500
⋅
50E
8
−
3
=
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
⋅
Rdivider
√1+(2⋅π⋅f⋅Rdivider ⋅ Cdivider)2
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:
Ft
=
2
⋅
π
⋅
1
√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:
2
⋅
π
⋅
1
Ft ⋅
R1 ⋅
C
=
√
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 :
5/13
5 Page 
AN1013 APPLICATION NOTE
Figure 18. Effect of inductance and Capacitance on Damping
Output
Underdamped
Overdamped
Fig.4
Frequency
Effect of Inductance and Capacitance on Damping
If a sharper cutoff is preferred, to attenuate better the carrier frequency Butterworth filters of the 3rd or
of the 4rth order could be used.
For example with a 3rd order filter (fig.19) the obtained frequency response is shown in fig.20.
The equations used to domension the passive components are the following:
L1
=
0.5 ⋅
2π
Rload
⋅ Fc
L2
=
1.5 ⋅ Rload
2π ⋅ Fc
C1
=
2π
⋅
1.33
Rload
⋅
Fc
For Rload = 8ohm, Fc = 30Khz resulted L1 = 20µH, L2 = 63µH, C = 0.88µF
Figure 19
Figure 20. 3rd order Butterworth filter
L2 L1
C1 RL
Components choice:
Not only the right choice of the LC network values is important, but it is also the type of the passive com-
ponents in the Class-D amplifier performances evenmore in terms of Losses and Harmonic distortion.
CAPACITOR: The losses in the filter capacitor due essentially to the ESR will be neglegible if Multilayer
film capacitors are used. Multilayer Mylar, Polypropilen or Policarbonate film capacitors are the re-
comended choice, we advise to avoid using ceramic capacitors for such kind of application.
INDUCTOR: The losses in the inductor at low frequencies are mainly due to the coil winding series re-
sistance. At higher frequencies, where the Skin Effect becomes of importance, a multiwire winding could
be effective to obtain the maximum in terms of efficiency. However for most of the applications the use
of a single wire winding with adequate cross section is enough.
The inductor used for the filter must sustain a DC current greater than the specified current limitation
value (see the datasheet of the device of interest for the value to consider) without saturation and the
coil material must show very low hysteresis losses.
11/13
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