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AN726

UTILITY CLASS-D TOOLSTICK USER’S GUIDE

Silicon Laboratories 

AN726

6. Tuning the Class-D Output Network

6.1. Revision 1.0 Configuration

Class-D is an extension of a single-ended, single-pin speaker drive, where one side of the speaker is driven

directly (perhaps with a ferrite bead) and one side is grounded. This output method works very well with SiM3U1xx

devices.

The full Class-D output network consists of ferrite beads, inductor-capacitor (LC) filters, and grounding capacitors

(Cg). Figure 20 shows the output network configuration used by the Revision 1.0 hardware. An alternate output

network shown in Figure 21 may provide better performance than the original configuration.

PB4.0 (R+)

FB

L

Cg

C

PB4.1 (R-)

FB

L

Cg

Figure 20. Revision 1.0 Hardware Class-D Output Network—Direct Drive

The ferrite beads dissipate pad switching noise as heat for EMI purposes, the LC components filter the PWM noise,

and the grounding capacitors provide additional filtering in conjunction with the speaker reactance.

In almost all applications, the ferrite beads should be populated on the board. However, the other components may

or may not be populated, depending on the speaker used and the cost requirements of the application. If either the

L or the C are installed, both should be populated to ensure lower power consumption. In most cases, the easiest

method of determining whether to install the components and the values for these components is through physical

experimentation. The perceived quality can be determined by the following characteristics:

loudness

changes in loudness across bandwidth

bandwidth

noise harmonics across bandwidth

hiss at low and high volumes

Other factors to consider are the application’s sound source (i.e. USB data versus sampled ADC data) and the

maximum power consumption of the board in the application, which will determine how much data processing can

occur and if external MOSFETs are appropriate. The Class-D board provides 0  resistor pads to bypass the

series components, if desired.

18

Rev. 0.1

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