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Texas Instruments Post Filter Feedback Class-D Amplifier Benefits and Design Considerations Application notes
Application Report
SLOA260 – November 2017
Post Filter Feedback Class-D Amplifier Benefits and
Design Considerations
Jasjot Chadha
ABSTRACT
This document details post filter feedback (PFFB) technology from Texas Instruments. It will guide
designers through the audio system benefits that can be realized by implementing this technology
inclusive of performance and cost.
1
2
3
Contents
Introduction ................................................................................................................... 2
Post Filter Feedback (PFFB) ............................................................................................... 2
Loop Stability With PFFB ................................................................................................... 3
List of Figures
1
Class-D Loop With PFFB ................................................................................................... 2
2
TAS2560 THD+N vs Frequency Comparison ............................................................................ 3
3
Ferrite Bead Equivalent Model ............................................................................................. 3
4
Impedance Curve for MPZ1608S221A
1
Comparison of Inductive Filters ............................................................................................ 2
2
PFFB Sability Criteria for TAS25xx Series ............................................................................... 4
...................................................................................
3
List of Tables
Trademarks
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All other trademarks are the property of their respective owners.
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Post Filter Feedback Class-D Amplifier Benefits and Design Considerations
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1
Introduction
1
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Introduction
A Class-D audio amplifier output is a switching output that has a carrier frequency along with the audio
signal content. These switching frequencies can lead to system design challenges with EMI radiation due
to long traces running to the speaker loads. In order to meet EMI radiation specs like FCC/CE, the ClassD amplifier outputs typically have EMI filters realized using LC filters at the output. The inductor in the filter
can be implemented using a ferrite bead, which is more cost optimized than a regular inductor, but comes
with its own non-linearities. Using cost-optimized ferrite beads can degrade the THD performance of the
audio after the filter, affecting the audio performance of the end system. Such concerns often lead
designers to go back to an inductor based filter or use a more expensive ferrite bead which does not
degrade the THD performance but ultimately increases the cost of the end system. Another concern with
using a filter at output of a Class-D amplifier is that it degrades the frequency response of the output
depending upon the cut-off frequency selected for the filter. This also affects the audio performance
across frequencies. Table 1 lists the cost of different inductors and ferrite beads. The PFFB configuration
from Texas Instruments™ allows usage of low cost ferrite beads without the performance degradation
associated with these components.
Table 1. Comparison of Inductive Filters (1)
(1)
2
Part Number
Type
9 k Unit Price
Performance
MAKK2016TR24M
Inductor
$0.071
High
NFZ2MSM181SN10L
Ferrite bead
$0.111
High
MPZ1608S221A
Ferrite bead
$0.018
Low
Sourced from mouser.com.
Post Filter Feedback (PFFB)
TI’s PFFB allows the user to add the ferrite bead inside the Class-D loop as shown in Figure 1. This
change in loop configuration ensures that the errors added by ferrite beads are added inside the Class-D
loop and will get corrected by the loop gain. This coupled with the high loop gain that comes with TI’s
fourth order Class-D amplifiers in devices such as TAS2560, TAS2557, and TAS2559 amplifiers ensures
that THD degradation due to the ferrite bead addition are minimized. This allows designers to use cost
optimized ferrite beads without worrying about the adverse effects of these beads on their system’s audio
performance. Also, since the loop is closed after the ferrite beads filters, there is no dip in the frequency
response of the amplifier which results in giving the amplifier a flat amplitude response across the audio
band of frequencies.
INPUT
_
+
Loop
Filter
Ramp
Generator
H-Bridge
and
Driver
To Speaker
RAMP
EMI Filter
Figure 1. Class-D Loop With PFFB
2
Post Filter Feedback Class-D Amplifier Benefits and Design Considerations
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Loop Stability With PFFB
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Figure 2 compares the THD+N vs Frequency for TAS2560 with MPZ1608S221A ferrite bead filter coupled
with a 100-pF capacitor. As it is seen in Figure 2, the THD degrades significantly at higher frequencies
when the ferrite bead is kept outside the Class-D loop like in traditional Class-D amplifiers. With TI’s PFFB
configuration, the performance degradation due to ferrite bead is greatly reduced.
10
5
Without ferrite bead
Loop closed after ferrite bead(PFFB)
Loop closed before ferrite bead
THD+N(%)
2
1
0.5
0.2
0.1
0.05
0.02
0.01
0.005
0.002
0.001
20 30 50
100 200
500 1000 2000
Frequency(Hz)
10000
50000
D006
Figure 2. TAS2560 THD+N vs Frequency Comparison
3
Loop Stability With PFFB
Because the ferrite bead filter is now inside the Class-D loop, there are extra poles added in the system
which adversely affect the stability of the loop. The designer must take into account extra guidelines in
order to choose the correct configuration of the filter to ensure the Class-D loop is stable.
To find a stable configuration of the EMI filters, the equivalent model of the EMI filter needs to be drawn.
The filter must be approximated into a second order filter as shown in Figure 3.
RPAR
RDC
LEQ
CEQ
CPAR
Figure 3. Ferrite Bead Equivalent Model
Ferrite beads impedance can be divided into three main regions (for example: inductive, resistive, and
capacitive). These regions can be easily determined by looking at impedance plots of ferrite bead data
sheet (shown in Figure 4), where Z is the impedance, X is the reactance, and R is the resistance of the
bead.
Impedance (Ω)
300
250
Region 2
Region 1
200
Z
150
R
100
50
0
X
1
10
100
1000
Frequency (MHz)
10000
Figure 4. Impedance Curve for MPZ1608S221A
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Loop Stability With PFFB
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For the region of the impedance plot where the bead appears mostly inductive (region 1 in Figure 4), the
LEQ can be calculated by using Equation 1. It is best to take the impedance value at least a decade away
from the peak impedance value for accurate calculations. For example, impedance seen in Figure 4 at 10
MHz for the MPZ1608S221A is 70 Ω. LEQ can be calculated as 1.11 µH.
XL
2u Su f
LEQ
where
•
XL = impedance of the bead at frequency (f)
(1)
The CPAR can be estimated in a method similar to LEQ, by looking at region where the bead appears mostly
capacitive (region 2 in Figure 4). The CPAR can be estimated using Equation 2. It is best to take impedance
value at least a decade away from the peak impedance value for accurate calculations. For most beads,
the CPAR is less than 5 pF and has no impact on the stability of the loop. User should do the calculations
and ensure that this holds true for the bead selected for the use case. For example, impedance seen in
Figure 4 at 1 GHz for MPZ1608S221A is 150 Ω. CPAR can be calculated as 1 pF.
CPAR
1
2 u S u f u XC
where
•
XC = impedance of the bead at frequency (f)
(2)
RPAR can be approximated as the peak impedance of the bead. For ease of calculations, RDC is
approximated as zero here, and the entire peak impedance is estimated as RPAR. In Figure 4, RPAR can be
calculated as 250 Ω.
The total output capacitance (CEQ) should include the intentional capacitance added by the user for the
filtering and the parasitic capacitors due to any other additional elements at output of the ferrite bead such
as ESD diodes, board routings, and more.
Using the model of the filter, the filter cutoff frequency and the Q-factor can be calculated by using
Equation 3 and Equation 4.
1
Z0
2S u LEQ u CEQ
Q
RPAR u
(3)
CEQ
LEQ
(4)
Table 2 summarizes the stability criteria for TI’s PFFB, Class-D amplifiers. It is recommended that
designers ensure the stability criteria for the selected filter satisfy the guidelines in Table 2 for proper
functioning of the Class-D amplifier.
Table 2. PFFB Sability Criteria for TAS25xx Series
4
Part No.
Cutoff Frequency – ω0
ω0 / Q
TAS2560
> 10 MHz
> 2e6
TAS2557
> 10 MHz
> 2e6
TAS2559
> 10 MHz
> 2e6
Post Filter Feedback Class-D Amplifier Benefits and Design Considerations
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Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Date
Revision
Description
November 2017
*
Initial release.
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Revision History
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