TI Information – Selective Disclosure Click to edit Master subtitle style Audio Amplifier Design Tips May 2012

TI Information – Selective Disclosure

Click to edit Master subtitle style

Audio AmplifierDesign Tips

May 2012


Class A/B

Class D

Easy design

Simple PCB – 1 or 2 layers

Fewer components

No EMI

Better sound quality

Better efficiencySmaller size

Why Class A/B?

LM4780Stereo A/B 60W

StereoClass D 50W


FeaturesParallel operation boosts the available output current and is valuable when driving low impedance loads.Output ballast resistors are needed to make sure the amplifiers are evenly loaded.

Parallel Operation


FeaturesBridge operation doubles the output voltage swing on the same supple.The result is up to 4 times the output powerThis circuit show how to configure the LM3886 for bridge operation

Bridge Operation

44


Oscillation

Solutions

Oscillation can develop for many reasons.The scope photo shows some “fuzz” on the lower side of the sine wave

Oscillation may also occur at all points on the sine wave.

SnubberA simple R/C filter on the output will usually fix a bottom side oscillation

Amplifier gainMost high power audio A/B amplifiers require a voltage gain larger than 10 for stability.Filter across the feedback resistor may lower the gain and cause oscillationPower SupplyBypass caps close to the device.

StabilityWhat to look for

55

Snubber

Av > 10

Bypass Caps


Power Dissipation

Thermal Resistance

All IC’s dissipate power to some degreeAudio power amplifiers generate a significant amount of hearPower dissipation varies depending on:Power supply voltageOutput load – 8 or 4 ohms

Determined by the path the heat takes to get “out” of the packageqja is referred to as “junction-to-ambient”qjc is referred to as “junction-to-case”Heat-sinks also have a thermal resistance specified in Degrees C/W.

Thermal ConsiderationsWhat factors are involved

66


Determine the operating conditions

Use the PDMax equation

LM1875 datasheet

Supply voltage = +/- 25V

Load = 8 ohms

PDMax = V2(supply total)/(2*p2RLoad) + PQ

Thermal ConsiderationsHow to calculate power dissipation – LM1875

77

CalculatePDMax for LM875

PDMax = (50v)/(2*(3.14)2*8) + (50v*70Ma)

PDMax = 15.85 + 3.5 = 18.85W


The Easy Way LM1875

Most datasheet supply a “power dissipation” curve

This is the easy way to determine PDMAX

However, not all condition may be included.

Make sure to pick the correct graph for the load

Find the curve for the Supply Voltage

Locate PDMAX

Thermal ConsiderationsPower Dissipation Curves

88


Thermal Resistance

LM1875

The total thermal resistance must be calculated

(LM1875 Qjc + Heat Sink Thermal Resistance)

= (3oC/W + 2oC/W) - assume heat-sink of 2oC/W

= 5oC/W

Thermal ConsiderationsHow hot will the device get?

99

Max Device Temperature

Assuming a max ambient temperature of 50 deg C, the max device

temperature can be calculated

(Thermal resistance)*PDMAX + T(MAX AMBIENT)

= (5oC/W) * (18.85W) + 50oC = 144oC

Note: Max temp may not exceed 150oC


The Easy WayLocate PDMAX on the vertical axis

Locate the max ambient temperature on the horizontal axis

Pick the appropriate heasink thermal resistance

Note all lines intersect at a max IC junction temperature of 150oC

Thermal ConsiderationsPower de-rating Curves

1010


Current Flow

Where is the input ground

In this case the input ground is connected to the outputThe signal on the output ground is now transferred to the input groundThis is effectively another signal injected into the input of the amplifier.

PCB LayoutGround Trace Routing

1111

Large current flows from the Output Groundto the power supply ground (Blue Arrow)

The trace connecting the two grounds is large, but still has resistance.This current flow generates a voltage waveform


Setup

Analysis

Connect the amplifier load and power supplyConnect the amplifier input and output to a distortion analyzer.Connect one scope probe to the amplifier outputConnect the “reading” output of the distortion analyzer to another scope input

The amplifier output signal is shown on the right with the yellow trace.The “reading” output is shown in green.The reading trace represents what the analyzer is actually measuringThis particular amplifier has a grounding issue caused by improper connections of the input ground as shown in the last slide

PCB LayoutHow to Evaluate

1212


The FixThe input ground is now disconnected from the

output ground.The ground is routed to the quiet ground (Cap Ground)

PCB LayoutGround Trace Routing

1313


Proper Operation

This is the same amplifier as shown in the

previous slide

Grounding problem solved

Notice the low distortion levels

Dominant factor is crossover distortion

PCB Layout

1414


Audio Power Amplifier Roadmap

LM38861x70W Overture

LM3875,761x60W Overture



LM4781,23x35W Overture

LM47022x125W Driver

LM18751x30W

LM4752,552x11W

LM49502x3, 1x10W

LME498101x400W Driver

Relative Performance

1515

Mid Power0.02% THD

OvertureSPiKe Protection

0.002% THD

LME Series0.0005% THD


Types of Protection

Current Limiting

Thermal Shutdown – turn off the device if it gets too hot

Current limiting – clamp the output current when

it gets too large

SOA (Safe Operating Area) Protection – limit the

power dissipated in the output transistors

Current flow through RE to the load

This causes V to rise in value

When V reaches about 0.7 volts, I begins to flow

I pulls the base drive from the output transistors,

limiting the output current

Output Protection

1616

CurrentFlow

VI


Features

Beneifits

Current LimitingOvervoltage ProtectionSPiKe ProtectionSelf PeakInstantaneousTemperature (Ke)

Built into the output transistorsActs instantlyMonitors all portions of the output transistors

Output ProtectionOverture SPiKeTM Protection

1717

Beneifits

Overture power amps do not fail


Topoloogy

Benefits

The “Driver”, red box, includes:

Pre-amp

Mute

Compensation

Baker clamp

Power transistors , blue box, are external

High voltage operation – up to 200V

Scalable output power

Add more output transistors

Low distortion – 0.0005%

Audio Power Amp Drivers

1818


Biasing

VBE on the output transistors changes with temp

Optimum output bias current must be maintained

A VBE multiplier, red outline, is used.

QMULT is mounted next to the output transistors

QMULT is at the same temperature

QMULT’s VBE tracks the output transistors and

maintains a constant bias current

Audio Power Amp Drivers

1919

VBE = 0.7VNominal

VBias = 0.7V * (RB2/(RP+RB1))


Summary (Conclusion)

•Circuit design Considerations–Stability–Thermal

•PCB design considerations–Grounding

•Output Protection–SPiKe

•High Voltage Audio PA Driver (200V)

2020


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TI Information – Selective Disclosure Click to edit Master subtitle style Audio Amplifier Design Tips May 2012