dm00129410

AN4569
Application note
Using Sound Terminal ® STA3xyBWz devices
in 2.1-channel applications
Stefano Bosco
Introduction
The purpose of the document is to show different possibilities of using STA3xyBWz devices
in 2.1-channel applications.
The first part of the document is dedicated to a single device solution while the second part
of the document is dedicated to a solution using two devices.
October 2014
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www.st.com
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Contents
AN4569
Contents
1
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2
2.1-channel application using one device . . . . . . . . . . . . . . . . . . . . . . . 6
2.1
2.2
3
2.1.1
Component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.2
Decoupling capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.3
Output filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.4
Snubber network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1.5
Main filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1.6
EMI filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.7
DC cut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Software configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1-channel applications using two devices . . . . . . . . . . . . . . . . . . . . . 13
3.1
3.2
4
Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.1.1
Component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1.2
Decoupling capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1.3
Output filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1.4
Snubber network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1.5
Main filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1.6
Damping network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.1.7
EMI filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Software configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1
Decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.2
Snubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.3
PLL filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.4
Audio performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.5
Thermal dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
6
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
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List of tables
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Component values for 2.1-channel, single-device applications . . . . . . . . . . . . . . . . . . . . . 10
DC cut - resistor and capacitor values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Component values for 2.1-channel applications, two devices . . . . . . . . . . . . . . . . . . . . . . 18
2.1 channels, two devices - damping network values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Document revision history. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
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List of figures
AN4569
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.
Figure 35.
4/29
2.1-channel application with one device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Schematic for decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
THD + Noise ratio with different values of the tank capacitor . . . . . . . . . . . . . . . . . . . . . . . . 8
Schematic for output filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Schematic for snubber section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Schematic for main filter section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Schematic for EMI filter section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Principle schematic for DC cut section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Schematic for DC cut section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Schematic of stereo BTL configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Schematic of mono BTL configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Schematics for decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
THD+Noise ratio with different values of the tank capacitor . . . . . . . . . . . . . . . . . . . . . . . . 15
Schematics for output filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Schematics for snubber section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Schematics for main filter section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Schematics for damping network section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Schematics for EMI filter section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Electrolytic capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
VCC - GND paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Star connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
VCC filtering for high frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Correct layout for decoupling capacitors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Snubber network. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Snubber filter for spikes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Differential and single-ended layout for snubber network. . . . . . . . . . . . . . . . . . . . . . . . . . 23
PLL filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Symmetrical paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Coil separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Different GND planes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Output routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Good and bad output routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Thermal dissipation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Thermal layout for top and bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Thermal dissipation - holes under IC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
DocID026803 Rev 1
AN4569
1
Overview
Overview
In the LCD-TV market the tendency to constantly reduce the thickness of thin-screen TVs
forces speaker manufacturers to design new components with very small dimensions.
The result of this process negatively impacts the quality of the sound reproduction because
the frequency response of the full range is very narrow and the sensitivity is quite low.
To reduce this issue, in high-quality LCD-TV sets the speaker system is often implemented
using a 2.1-channel active solution: 2 channels are dedicated to reproduce the mid and high
frequencies (for both left and right channels) while the lowest portion of the audio band,
usually below 200 Hz, is reproduced using a single large speaker (subwoofer channel)
enclosed in a suitable box. Each speaker is driven by a dedicated power amplifier (3 in total)
and the crossover filters are implemented using the DSP already present in the audio chain.
Although this solution should provide good performance, the audio quality the final user
perceives is not optimal because of the inferior quality of the speakers assembled in the TV
set.
All Sound Terminal® devices for the STA3xyBWz family can use different output
configurations.
The standard application for this kind of device is a stereo BTL configuration for left and right
channels. There are other possibilities:
1. Two BTL channels (left and right) + third channel for external bridge;
2. Two single-ended for left and right channels + third channel in BTL;
3. Mono BTL configuration.
With this scenario it is possible to have a three-channel output using one or two devices.
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2.1-channel application using one device
2.1
Schematic
AN4569
Figure 1. 2.1-channel application with one device
In this configuration only one device has been used. The CONFIG pin must be connected to
GND.
2.1.1
Component selection
The selection of components is a key factor for the cost of the entire application.
The following list includes components grouped according to different sections of the
schematic:
-Decoupling capacitors on VDD_DIG and PLL_VDD pins are 100 nF low voltage;
-Decoupling capacitors on VCC pins (100 nF, 1 uF and 100 uF) must be aligned with the
external power supply used. If 24 V is used, the rate for the capacitors must be 35 V;
-Decoupling capacitors on the internal regulator (C6 and C7 in the schematic) can have a
low voltage rate. This because they are 3.3 V regulators;
-Coil saturation current is compatible with the peak current of the application;
-The size of the resistor divider (R5, R6, R7 and R8) must be selected according to the VCC
selected;
-The rate for the capacitors divider (C19, C20, C21 and C22) must be selected according to
the VCC selected;
-The size of the resistor and capacitor dividers must be selected according to the output
load and desired curve response. In Section 2.1.7: DC cut, there is a detailed explanation.
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2.1.2
2.1-channel application using one device
Decoupling capacitors
Figure 2. Schematic for decoupling
The decoupling capacitors on high-voltage pins (C24, C25, C26 and C27) must be placed
as close as possible to the VCC pins. This is in order to avoid parasitic induction with the
copper wires on the PC board. For each VCC pin, two capacitors are needed
(100 nF and 1 μF). The meaning of each decoupling component is as follows:

C23 is a part of the power supply and it is used as a far tank capacitor.

The 100 nF capacitors (C24 and C25) smooth the high-frequency spikes

The 1 μF capacitors (C26 and C27) smooth the low-frequency spikes and they are also
used as a close tank for the 100 nF capacitor.
The decoupling capacitors on the VDD_DIG and PLL_VDD pins (C1, C2 and C3) are placed
close the pins.
The reason that the VDD_DIG and PLL_VDD pins are separated with beads is to avoid high
frequencies on VDD_DIG. It is possible to eliminate these components if the layout is done
with a right star connection.
The decoupling capacitors on the regulators (C6 and C7) are also placed close to the
device.
All decoupling capacitors must be X7R. This dielectric is preferable because the X7R
capacitor is stable over temperature changes (±15%).
For example, the Y5V dielectric is very unstable (specs are +22% -82%).
The following figure shows the THD at different values of the tank capacitor (C23) in the ST
demo board using the bench power supply. The minimum THD is similar using10 μF or
1000 μF capacitors. The main difference is at medium-high power where the tank is not
enough to guarantee the peak current.
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2.1-channel application using one device
Figure 3. THD + Noise ratio with different values of the tank capacitor
2.1.3
Output filter
Figure 4 shows the output filter scheme for a 2.1-channel application.
Figure 4. Schematic for output filter
The output filter is composed of three different sections:

Snubber network

Main filter

EMI filter
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2.1.4
2.1-channel application using one device
Snubber network
Figure 5. Schematic for snubber section
The function of a snubber network is to absorb energy from reactance in the power circuit.
The purpose of the RC snubber is to avoid unnecessary high pulse energy such as a spike
in the power circuit which is dangerous to the system. By using the snubber network, the
energy (big spike) is transferred within the snubber network, allowing safe operation of the
system.
The snubber network is a part of the schematic that is layout dependent. The values shown
in the schematic (R = 22 ohm, C = 330 pF) represent the best solution for the ST demo
board.
In Figure 5 there are two types of snubber: BTL and single-ended.
Differential snubber
The power on this network is dependent on the power supply, frequency and capacitor value
using the following formula:
PDiff = C * f * (2 * VCC)2
The power must be dissipated on the series resistor.
Single-ended snubber
This type of snubber is used to increase the efficiency or, otherwise, must be used if the
output configuration is single-ended. In a single-ended snubber the power on the network is:
Equation 1
2.1-channel single device - power on snubber
PSE = C * f * (VCC)2
PSE = PDiff / 4
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2.1-channel application using one device
2.1.5
AN4569
Main filter
Figure 6. Schematic for main filter section
The purpose of the main filter is to cut off the frequency above the audible range of 20 kHz.
The main filter is designed using the Butterworth method to define the cutoff frequency. In
order to have a clean amplifier, a cutoff frequency above 20 kHz is mandatory.
For class-D amplifiers, the output load is a part of the filter. The following formulas are used
to verify the values of the LC components.
Equation 2
Main filter equation for SE and BTL
R load
1
C load = C11 = C12 = C13 = ---------------------------------------------------- L load = L3 = L4 = ---------------------------------------------2    R load  f cutoff
2    2  f cutoff
R load
L SE = L1 = L2 = ------------------------------------  2  f cutoff
1
f cutoff = ---------------------------------------------------------------2    2  L load  C load
Following table show typical component value for standard output load.
Table 1. Component values for 2.1-channel, single-device applications
Rload
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8Ω
6Ω
4Ω
3Ω
L1, L2
47 μH
33 μH
22 μH
15 μH
C11, C12
390 nF
470 nF
680 nF
1 μH
L3, L4
22 μH
15 μH
10 μH
10 μH
C13
470 nF
680 nF
1 μF
1 μF
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2.1.6
2.1-channel application using one device
EMI filter
Figure 7. Schematic for EMI filter section
The purpose of the EMI filter is to avoid radiated emission. It is composed of two 1 nF
capacitors that must be connected to GND. The values of these capacitors must be trimmed
in the application.
2.1.7
DC cut
Figure 8. Principle schematic for DC cut section
Figure 9. Schematic for DC cut section
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AN4569
In single-ended applications the output signal must vary between -Vcc/2 and +Vcc/2 instead
of -Vcc +Vcc as in BTL mode. For this reason a dedicated electrolytic capacitor must be
connected between the PWM output and speaker as shown in Figure 8.
The purpose of the resistive divider (R5 and R6 for OUT1, R7 and R8 for OUT2) is to take
the output at half the supply when the device is in tri-state. The aim is to charge slowly the
decoupling capacitor at the turn-on of the power supply in order to avoid “pops”.
The value of the electrolytic capacitor is related to the load impedance and the lower limit of
the audio bandwidth. The value is determined using the following formula:
Equation 3
Series capacitor value in SE configuration
1
C 19 = C 21 = -----------------------------------------------2    f 3dB  Z spkr
To improve the audio performance (pop and noise) and to reduce the size of components, it
is possible to split the electrolytic capacitor into two capacitors of equal value (half the
value), as shown in Figure 9.
In this case the output speaker is not connected to GND, but it is connected to the same
potential as the PWM output (virtual GND). To avoid a "pop", the bridge must be muted for 4
or more R * C time constant where R = R7 and C = C22.
The recommended schematic is shown in Figure 9.
Table 2. DC cut - resistor and capacitor values
Rload
2.2
8Ω
6Ω
4Ω
3Ω
R5, R6, R7, R8
6.2 kΩ
4.7 kΩ
3.3 kΩ
2.7 kΩ
C19, C20, C21, C22
82 μF
100 μF
180 μF
220 μF
Software configuration
In order to set the device in 2.1-channel configuration, change the register configuration
0x05 (configuration register F) from 0x5C (default) to 0x5D.
To switch on the power bridge, set to one the bit D7 of register 0x05.
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2.1-channel applications using two devices
3
2.1-channel applications using two devices
3.1
Schematics
Figure 10. Schematic of stereo BTL configuration
Figure 11. Schematic of mono BTL configuration
In this configuration, two devices must be used.
The I2C address pin (SA) must be set to 0 in one device and to 3.3 V in the other.
The CONFIG pin must be connected to GND for the device that is configured in stereo BTL
mode and it must be connected to Vdd (pin 15) for the device that is configured in mono
BTL.
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2.1-channel applications using two devices
3.1.1
AN4569
Component selection
The selection of components is a key factor for the cost of the whole application.
The following list includes components grouped according to different sections of the
schematic:

Decoupling capacitors on VDD_DIG and PLL_VDD pins are 100 nF low voltage;

Decoupling capacitors on VCC pins (100 nF, 1 μF and 100 μF) must be aligned with the
external power supply used. If 24 V is used, the rate for the capacitors must be 35 V;

Decoupling capacitors on the internal regulator (C6S, C6M, C7S, C7M and C14M in
the schematic) can have a low-voltage rate. This because they are 3.3 V regulators;

Coil saturation current must be compatible with the peak current of the application.
3.1.2
Decoupling capacitors
Figure 12. Schematics for decoupling
The decoupling capacitors on high-voltages pins, (C21S/M, C22 S/M, C23 S/M and C24
S/M) must be placed as close as possible to the VCC pins. This is in order to avoid parasitic
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2.1-channel applications using two devices
inductance with the copper wires on the PC board. For each VCC pin, two capacitors are
needed (100 nF and 1 μF). The meaning of each decoupling component is as follows:

C23S and C24M are part of the power supply and they are used as a far tank capacitor.

The 100 nF capacitors (C24S, C24M, C25S and C25M) smooth the high-frequency
spikes

The 1 μF capacitors (C26 and C27) smooth the low-frequency spikes and they are also
used as a close tank for the 100 nF capacitor.
The decoupling capacitors on the VDD_DIG and PLL_VDD pins (C1, C2 and C3) are placed
close to the pins.
The reason why the VDD_DIG and PLL_VDD are separated with beads is to avoid high
frequencies on VDD_DIG. It is possible to avoid these components if the layout is done with
the right star connections.
Decoupling capacitors on regulators (C6 and C7) are placed close to the device.
All decoupling capacitors must be X7R. This is because the X7R capacitor is stable over
temperature (±15%).
For example, the Y5V dielectric is very unstable (specs are +22% -82%).
The next figure shows THD at different values of the tank capacitor (C23) in the ST demo
board using the bench power supply. The minimum THD is similar using 10 μF or 1000 μF
capacitors. The main difference is at medium-high power where the tank is not sufficient to
guarantee the peak current.
Figure 13. THD+Noise ratio with different values of the tank capacitor
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2.1-channel applications using two devices
3.1.3
AN4569
Output filter
Figure 14 shows the output filter scheme for a 2.1-channel application.
Figure 14. Schematics for output filter
The output filter is composed of three different sections:

Snubber network

Main filter

EMI filter
3.1.4
Snubber network
Figure 15. Schematics for snubber section
The function of a snubber network is to absorb energy from reactance in the power circuit.
The purpose of the RC snubber is in order to avoid unnecessary high pulse energy such as
spike in power circuit which is dangerous to the system. By using the snubber network, the
energy (big spike) is transferred within the snubber network, allowing safe operation of the
system.
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2.1-channel applications using two devices
The snubber network is a part of schematic that is layout dependent. The values shown in
the schematic (R = 22 ohm, C = 330 pF) represent the best solution for the ST demo board.
In Figure 15 there are two types of snubber: BTL and single-ended. It is possible to select a
differential or single-ended snubber according to the power dissipation on the resistor and
within layout constraints.
Differential snubber
The power on this network is dependent on the power supply, frequency and capacitor value
using the following formula:
P Diff = C  f   2  V CC 
2
The power must be dissipated on the series resistor.
Single-ended snubber
This type of snubber is used to increase the efficiency or, otherwise, must be used if the
output configuration is single-ended. For a single-ended snubber the power on the network
is given in the equation below.
Equation 4
2.1 channels, two devices - power on snubber
P SE = C  f   V CC 
2
P SE = P Diff  4
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2.1-channel applications using two devices
3.1.5
AN4569
Main filter
Figure 16. Schematics for main filter section
The purpose of the main filter is to cut off the frequency above the audible range of 20 kHz.
The main filter is designed using the Butterworth method to define the cutoff frequency. In
order to have a clean amplifier, a cutoff frequency above 20 kHz is mandatory.
For class D amplifiers, the output load is a part of the filter. The following formulas are used
to verify the values of LC components.
Equation 5
2.1 channels, two devices - main filter
1
C load = C11 = C12 = C13 = ----------------------------------------------------2    R load  f cutoff
R load
L load = L1S = L2S = L3S = L4S = L1M = L2M = ---------------------------------------------2    2  f cutoff
1
f cutoff = ---------------------------------------------------------------2    2  L load  C load
The following table shows the recommended values for the components.
Table 3. Component values for 2.1-channel applications, two devices
Rload
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8Ω
6Ω
4Ω
3Ω
L1S, L2S, L3S, L4S, L1M, L2M
22 μH
15 μH
10 μH
10 μH
C18S, C19S, C13M
470 nF
680 nF
1 μF
1 μF
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3.1.6
2.1-channel applications using two devices
Damping network
Figure 17. Schematics for damping network section
The purpose of the damping network is to avoid the high-frequency oscillation issue on the
output circuit. After using the damping network the THD can be improved and the damping
network can also avoid the inductive copper on the PCB routing when the system is working
on high frequency with the PWM or PCM.
The C-R-C is mainly intended for high inductive loads.
The following table shows the recommended values for the components.
Table 4. 2.1 channels, two devices - damping network values
Rload
8Ω
6Ω
4Ω
3Ω
C10S, C11S, C12S,
C13S, C9M, C12M
100 nF
100 nF
220 nF
220 nF
C14S, C15S, C16S,
C17S, C10M, C11M
100 nF
100 nF
220 nF
220 nF
6.2
4.7
3.3
2.7
R4S, R5S, R6S, R7S,
R3M, R4M
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2.1-channel applications using two devices
3.1.7
AN4569
EMI filter
Figure 18. Schematics for EMI filter section
The purpose of the EMI filter is to avoid radiated emission. It is composed of three
capacitors: two 10 nF capacitors must be connected to GND and one 1nF capacitor must be
connected parallel to the speaker. The values of this capacitor must be trimmed in the
application.
3.2
Software configuration
No SW modifications are needed for the device that works in stereo BTL mode.
For the device that works in mono BTL mode, change the register configuration 0x05
(configuration register F) from 0x5C (default) to 0x5F to set to a single channel
configuration.
To switch on the power bridge, set to one the bit D7 of register 0x05.
To adjust the volume using a mono BTL device, change the channel 3 volume register
(address 0x0A).
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Layout
4
Layout
4.1
Decoupling

Use electrolytic capacitors first to separate the Vcc branches;
Figure 19. Electrolytic capacitor
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
Minimize the path between the Vcc pins and ground pin in order to avoid inductive
paths;
Figure 20. VCC - GND paths
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
Vcc routing. The purpose of a "star routing" for the Vcc supply is in order to avoid the
interference between different signals such as part A idle and another part B working
with full load. Under this condition the interference would evidently happen if no star
connection were implemented.
Figure 21. Star connection
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Layout
AN4569

Vcc filter for high frequency
Figure 22. VCC filtering for high frequencies
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The purpose of this filter is to avoid an inductive coil generated by the copper wire because
the system is working in PWM with fast switching (the frequency is about 340 kHz) so the
longer copper wire can easily become an inductor. To improve this we recommend using a
ceramic capacitor to balance the reactance. The ceramic capacitor must be placed as close
as possible to the related pins. The recommended distance between the capacitor to the
related pins is 5 mm maximum.

Decoupling capacitors
Solder the decoupling capacitors as close as possible to the related IC pin, the purpose of
which is to reduce the inductive coil with copper wire (parasitic inductor).
Figure 23. Correct layout for decoupling capacitors
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4.2
Layout
Snubber

Solder the snubber network as close as possible to the related IC pin;
Figure 24. Snubber network
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
Snubber filter for high-frequency spike on the PWM;
Figure 25. Snubber filter for spikes
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3:0DVFORVHDV
,&
SRVVLEOHWRWKH
SLQV DOVR FORVH
WRWKH
PLQXV
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As shown in the figure below, there are two different examples given (single-ended and
differential snubber networks).
Figure 26. Differential and single-ended layout for snubber network
A strong spike could happen if there is a long distance between the snubber network and
the pins, the IC could even be burnt by the big spike. It's recommended that the distance
between snubber network be within 3 mm, which takes into consideration the width of the
copper wire.
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Layout
4.3
AN4569
PLL filter

Solder the PLL filter as close as possible to the FILT pin;
Figure 27. PLL filter
3//ILOWHU
4.4
Audio performance

For differential applications create symmetrical paths for the output stage;
Figure 28. Symmetrical paths
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V\PPHWULFDO
SDWKV

Separate the coil in order to avoid crosstalk;
Figure 29. Coil separation
6HSDUDWHWKHFRLOVWR
DYRLGFURVVWDON
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Layout

Ground consideration of layout. To avoid interference between power ground and small
signal ground, the different ground planes must be separated as shown in the following
figure.
Figure 30. Different GND planes
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3RZHUJURXQGSODQHV

Output routing
Figure 31. Output routing
Figure 32. Good and bad output routing
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Layout
4.5
AN4569
Thermal dissipation

To dissipate heat, it is mandatory to have a big ground plane on both (top and bottom)
layers and solder the slug on the PCB;
Figure 33. Thermal dissipation
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
Thermal layout with big ground (1/3 for top and bottom layers)
Figure 34. Thermal layout for top and bottom layer
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7KHUPDOOD\RXWRQERWWRPOD\HU

Thermal layout with large ground (2/3 for thermal and soldering holes)
Figure 35. Thermal dissipation - holes under IC
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The thermal resistance junction in the bottom of the STA3xyBWz in order to maintain
ambient temperature is obtainable with a ground copper area of 4x4 cm and with 24 via
holes (refer to the example shown in the figure above).
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5
Conclusion
Conclusion
For 2.1-channel applications, the better choice in terms of audio quality is to use two
different ICs (one in stereo BTL mode and the other one configured in mono BTL).
If two ICs are selected, the mono BTL device must use an additional capacitor between the
GND_REG pin and GND plane.
If two ICs are selected, it is possible configure the output filter as filter-lite. Using a 2.1channel configuration with only one IC, all PWM outputs are configured in binary modulation
and for this reason it is not possible to use a filter-lite solution.
If it is mandatory to use all SMD components in the whole application (for example in TV
applications due to thickness constraints), the only possibility is to use two ICs because for
Left and Right channels it is mandatory to use two electrolytic capacitors for the channel.
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Revision history
6
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Revision history
Table 5. Document revision history
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Date
Revision
13-Oct-2014
1
Changes
Initial release.
DocID026803 Rev 1
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