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SLAA701A–October 2016–Revised November 2016
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Copyright © 2016, Texas Instruments Incorporated
LC Filter Design
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Application Report
SLAA701A–October 2016–Revised November 2016
LC Filter Design
ABSTRACT
In higher-power class-D amplifiers, generally above 10 W of output power, a filter on the output of the
amplifier is required. The filter is passive in nature and uses both an inductor and a capacitor on each
output terminal. Therefore, it is referred to as an LC filter. Proper component selection of the LC filter is
critical to meet the desired audio performance, efficiency, EMC/EMI requirements, and cost for the end
application. This application report serves as a guide to aid in the section of LC filter components for
class-D amplifiers to meet target-design goals of the end system.
Contents
1 Class-D Output Configurations............................................................................................. 3
1.1 Bridged-Tied Load (BTL) .......................................................................................... 3
1.2 Parallel Bridge-Tied Load (PBTL) ................................................................................ 3
1.3 Single-Ended (SE).................................................................................................. 4
2 Class-D Modulation Schemes.............................................................................................. 5
2.1 AD (Traditional) Modulation ....................................................................................... 5
2.2 BD Modulation....................................................................................................... 6
3 Class-D Output LC Filter.................................................................................................... 7
3.1 Output LC Filter Frequency Response Properties ............................................................. 7
3.2 Class-D BTL Output LC Filter Topologies....................................................................... 8
3.3 Single-Ended Filter Calculations.................................................................................. 9
3.4 Type-1 Filter Analysis............................................................................................. 10
3.5 Type-2 Filter Analysis............................................................................................. 12
3.6 Hybrid Filter for AD Modulation.................................................................................. 14
3.7 AD Modulation With Type-1 or Type-2 Filters ................................................................. 17
3.8 LC Filter Quick Selection Guide................................................................................. 17
4 Inductor Selection for High-Performance Class-D Audio ............................................................. 18
4.1 Inductor Linearity.................................................................................................. 18
4.2 Ripple Current ..................................................................................................... 20
4.3 Minimum Inductance.............................................................................................. 21
4.4 Core Loss .......................................................................................................... 22
4.5 DC Resistance (DCR) ............................................................................................ 23
4.6 Inductor Study With the TPA3251 Device ..................................................................... 24
5 Capacitor Considerations ................................................................................................. 28
5.1 Class-D Output Voltage Overview .............................................................................. 28
5.2 Capacitor Ratings and Specifications........................................................................... 30
5.3 Capacitor Types ................................................................................................... 35
6 Related Collateral ......................................................................................................... 40
List of Figures
1 Stereo (Two-Channel) BTL Class-D Amplifier ........................................................................... 3
2 Mono PBTL Class-D Amplifier ............................................................................................. 4
3 Four Single-Ended Outputs ................................................................................................ 4
4 AD (Traditional) Modulation ................................................................................................ 5
5 BD Modulation ............................................................................................................... 6
