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Transcript of TI Information – Selective Disclosure Click to edit Master subtitle style Audio Amplifier Design...
TI Information – Selective Disclosure
Click to edit Master subtitle style
Audio AmplifierDesign Tips
May 2012
TI Information – Selective Disclosure
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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Audio Power Amplifier Roadmap
LM38861x70W Overture
LM3875,761x60W Overture
LM28761x70W Overture
LM47802x60W 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
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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
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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
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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
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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))
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Summary (Conclusion)
•Circuit design Considerations–Stability–Thermal
•PCB design considerations–Grounding
•Output Protection–SPiKe
•High Voltage Audio PA Driver (200V)
2020
TI Information – Selective Disclosure
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