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Class D Amplifier

Class D Amplifier

Overview

Freescale's DSP5680x devices offer a combination of
peripherals and software to enable Class D amplifiers to work
at peak performance.

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Design Challenges

From the perspective of wasted power, the best amplifier
operates in pure switching mode-Class D. Normally, the
amplifier has efficiency higher than 90 percent, meaning over
90 percent of the power is delivered into the load. In fact, a
Class D amplifier works as a power D/A converter and can be
used to realize high output power, 300W or more. The Class D
amplifier is based on an analog technique of pulse width
modulation (PWM).

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Solution

Figure 1. Class D Amplifier Block
Scheme

As shown in the block scheme in Figure 1, an audio codec is
connected to the SPI of Freescale's DSP5680x, providing A/D
conversion of the input signal. The audio codec may be
bypassed and the SPI used directly as a digital input. Control
components, such as display and buttons, are connected to GPIO
pins. The most important component is the power stage
connected to the PWM output of the DSP and includes power
transistors, switchers of the output. The low pass filter is
passive and cuts frequencies exceeding the maximum frequency
of the input signal. Distortion and noise of the output signal
are leveled by cutting these high frequencies, presented in
the output signal because of the PWM modulation. The principle
of PWM modulation, illustrated in Figure 2, is quite
simple.

Figure 2. PWM Modulation
Principle

As shown in Figure 2, the input signal Vin
is modulated with modulation signal Vmod. There is a modulated signal in the
middle and the spectrum of the modulated signal is at the
bottom in the figure. The original signal fin
is selected with a low pass filter, cutting the higher
frequencies around the modulation frequency fmod.
The required frequency of PWM is 25 or more times faster than
the bandwidth of the signal being reproduced. A PWM modulation
frequency of 1MHz or higher should be used for 44,100Hz
sampling frequency of the audio codec.

Figure 3. D-Class Amplifier Software
Model

Figure 3 is a software model for an application designed to
be interrupt-driven. The most important feature is the
interrupt generated by the audio codec connected to the SPI.
Inside the interrupt routine, the data sample must be read and
processed. PWM Value Registers (PWMVAL) shown in Figure 2 are
updated from the input sample using the following formula:

PWM Value = (Input Data Sample) x (PWM
Modulus)

In the formula:

PWM Value = A new value of the PWM Value
register (PWM VAL)
Input Data Sample = Current
sample of input signal read from audio codec
PWM
Modulus = A value read from the PWM Counter Modulo
Register (PMCM)

To be displayed in the 16-bit PWM Value Register (PMVAL),
the result of the formula must be shifted.

The PWM module is configured to work in center-aligned
mode. The module will work in both independent and
complementary modes.
>

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Related Products

DSP56F801FA60 : Digital Signal Controller

The 56F801FA60, a member of the
56800 core-based family of Digital Signal
Controllers, combines the processing power of a
DSP ...

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Class D
Amplifier Documentation

Documentation

Fact
Sheets

ID and
Description

Vendor ID

Format

Size
K

Rev
#

Date
Last Modified

56F8000DSCFS
56F8000
Series Fact Sheet

FREESCALE

pdf

75

0

9/21/2006

Selector Guides

ID and
Description

Vendor ID

Format

Size
K

Rev
#

Date
Last Modified

SG2000CR
SG2000CR
Application Selector Guide Index and
Cross-Reference

FREESCALE

pdf

130

7

7/05/2005

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Class D Amplifier Design
Tools

Hardware Tools

Evaluation/Development Boards and Systems

ID and
Description

Vendor ID

Format

Size
K

Rev
#

Availability

56F800DEMO

56F800 Demonstration Kit

FREESCALE

-

-

-

56F801SLK

16-Bit DSP Student Learning Kit

FREESCALE

-

-

-

DEMO56F8013

Demonstration Board for 56F8013
Digital Signal Controller

FREESCALE

-

-

-

DEMO56F8014

Demonstration Board for 56F8014
Digital Signal Controller

FREESCALE

-

-

-

DSP56F801EVM

Evaluation Kit for 56F801 Processor

FREESCALE

-

-

-

DSP56F803EVM

Evaluation Kit for 56F803 Processor

FREESCALE

-

-

-

DSP56F805EVM

Evaluation Kit for 56F805 Processor

FREESCALE

-

-

-

DSP56F807EVM

Evaluation Kit for 56F807 Processor

FREESCALE

-

-

-

PBMCUSLK

Microcontroller Project Board

FREESCALE

-

-

-

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