Audio Power Amp. PCB LayoutClass-D design

By : Andri Haryono

Clasess in Audio Amplifier Audio Power amplifier circuits (output

stages) are classified as A, B, AB and C for analog designs, and class D and E for switching designs based upon the conduction angle or angle of flow, Θ, of the input signal through the (or each) output amplifying device, that is, the portion of the input signal cycle during which the amplifying device conducts

http://en.wikipedia.org/wiki/Electronic_amplifier

Class-D Class D – This class of amplifier is a switching

or PWM amplifier. This class of amplifier is the main focus of this application note. In this type of amplifier, the switches are either fully on or fully off, significantly reducing the power losses in the output devices. Efficiencies of 90-95% are possible. The audio signal is used to modulate a PWM carrier signal which drives the output devices, with the last stage being a low pass filter to remove the high frequency PWM carrier frequency.

Illustration

Advantages Reduce Heatsink size to minimum Low voltage requirement Theoritically High in efficiency (90-95%)

Disadvantages High cost High EMC, because the design need at

least 4 inductor

Comparison

PCB Layout forAudio Quality and EMC

Critical Circuit

Order of Priority Ground Plane Decoupling Placement and Connection EMC Circuit Placement and Connection Input and Analog Control Routing

Ground Plane

Ground Plane (Cont’d) Use ground plane to connect all critical

circuits This minimizes ground impedance,

especially

Ground Plane (Cont’d) Place the IC at the center of the ground plane

and ground the PowerPAD with thermal vias (0.33mm diameter on 1.0mm centers).

Ground decoupling caps and EMC components to the PowerPAD area through ground plane.

Place inputs on one side of the circuit and power and outputs on another to separate the currents.

This approach minimizes interference that could reduce audio quality and makes decoupling and EMC filters and snubers work best.

Decoupling Caps

Decoupling Caps (Cont’d) Place high-frequency decoupling caps within

1mm of the IC, and place bulk decoupling caps as close as possible to them.

Ground high-frequency decoupling caps and PGND pins to the PowerPAD.

Also ground decoupling caps and PGND pins to the ground plane through multiple vias.

This minimizes impedance and inductance in series with these capacitors.

This approach stabilizes power supply voltage and improves EMC by minimizing ringing.

Poor vs. Proper Decoupling

EMC Filtering

EMC Filtering (cont’d) Place EMC snubbers& filters very near the

IC. Ground through ground plane & connect

outputs to cap pads, directly or through broad copper, & not to inductors, to minimize stray inductance

Place EMC filter caps as close to the IC as possible on the ground plane layer and ground them to the IC through the ground plane

EMC Filtering (cont’d) This approach minimizes unfiltered

loops and trace lengths as well as stray inductance.

This makes EMC components function best & gives the widest possible filter bandwidth.

Poor vs. Proper Filter Placement

Inputs, Analog Control

Inputs, Analog Control Shield inputs with top layer ground

flood. Ground analog control caps (GVDD,

PLIMIT & AVCC) through ground plane with multiple vias.

Ground AGND pin directly to PowerPAD ts with top layer ground flood.

Reference Stephen Crump. Audio Power Amplifier

Applications. http://e2e.ti.com . Texas Instruments

Douglas Self. Audio Power Amplifier Design Handbook

Jun Honda & Jonathan Adams . Class D Audio Amplifier Basics

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