Audio Amplifier
Design Tips
Click to edit Master subtitle style
May 2012
TI Information – Selective Disclosure
Why Class A/B?
Class A/B
LM4780
Stereo A/B 60W
Easy design

Simple PCB – 1 or 2 layers

Fewer components

No EMI

Better sound quality

Class D
Better efficiency
Smaller size

TI Information – Selective Disclosure
Stereo
Class D 50W
Parallel Operation
Features
Parallel 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.

TI Information – Selective Disclosure
Bridge Operation
Features
Bridge operation doubles the output voltage
swing on the same supple.
The result is up to 4 times the output power
This circuit show how to configure the LM3886
for bridge operation

TI Information – Selective Disclosure
44
Stability
What to look for
Oscillation
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.

Solutions
Snubber
A simple R/C filter on the output will usually fix a
bottom side oscillation

Bypass
Caps
Amplifier gain
Most 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 oscillation
Power Supply
Bypass caps close to the device.

Snubber
Av > 10
TI Information – Selective Disclosure
55
Thermal Considerations
What factors are involved
Power Dissipation
All IC’s dissipate power to some degree
Audio power amplifiers generate a significant
amount of hear
Power dissipation varies depending on:
Power supply voltage
Output load – 8 or 4 ohms

Thermal Resistance
Determined by the path the heat takes to get “out”
of the package
qja 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.

TI Information – Selective Disclosure
66
Thermal Considerations
How to calculate power dissipation – LM1875
Determine the operating conditions
LM1875 datasheet

Calculate
PDMax for LM875
Supply voltage = +/- 25V

Load = 8 ohms

PDMax = (50v)/(2*(3.14)2*8) + (50v*70Ma)

Use the PDMax equation
PDMax = 15.85 + 3.5 = 18.85W

PDMax = V2(supply total)/(2*p2RLoad) + PQ

TI Information – Selective Disclosure
77
Thermal Considerations
Power Dissipation Curves
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

TI Information – Selective Disclosure
88
Thermal Considerations
How hot will the device get?
LM1875
Thermal Resistance
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
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

TI Information – Selective Disclosure
99
Thermal Considerations
Power de-rating Curves
The Easy Way
Locate 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

TI Information – Selective Disclosure
1010
PCB Layout
Ground Trace Routing
Current Flow
Large current flows from the Output Ground
to 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

Where is the input ground
In this case the input ground is connected
to the output
The signal on the output ground is now
transferred to the input ground
This is effectively another signal injected
into the input of the amplifier.

TI Information – Selective Disclosure
1111
PCB Layout
How to Evaluate
Setup
Connect the amplifier load and power supply
Connect the amplifier input and output to a distortion
analyzer.
Connect one scope probe to the amplifier output
Connect the “reading” output of the distortion
analyzer to another scope input

Analysis
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 measuring
This particular amplifier has a grounding issue
caused by improper connections of the input ground
as shown in the last slide

TI Information – Selective Disclosure
1212
PCB Layout
Ground Trace Routing
The Fix
The input ground is now disconnected from the
output ground.
The ground is routed to the quiet ground (Cap
Ground)

TI Information – Selective Disclosure
1313
PCB Layout
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

TI Information – Selective Disclosure
1414
Audio Power Amplifier Roadmap
LME Series
0.0005% THD
R
el
ati
ve
Pe
rfo
rm
an
ce
Overture
SPiKe Protection
0.002% THD
LM3886
1x70W Overture
Mid Power
0.02% THD
LM3875,76
1x60W Overture
LME49810
1x400W Driver
LM4702
2x125W Driver
LM4780
2x60W Overture
LM4781,2
3x35W Overture
LM2876
1x70W Overture
LM1875
1x30W
LM4752,55
2x11W
LM4950
2x3, 1x10W
TI Information – Selective Disclosure
1515
Output Protection
Types of Protection
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
I
Curre
nt
Flow
V
Current Limiting
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
TI Information – Selective Disclosure
1616
Output Protection
Overture SPiKeTM Protection
Feature
s Limiting
Current

Overvoltage Protection
SPiKe Protection
Self Peak
Instantaneous
Temperature (Ke)

Beneifits
Built into the output transistors
Acts instantly
Monitors all portions of the output
transistors

Beneifits
Overture power amps do not fail

TI Information – Selective Disclosure
1717
Audio Power Amp Drivers
Topoloogy
The “Driver”, red box, includes:

Pre-amp

Mute

Compensation

Baker clamp

Power transistors , blue box, are external

Benefits
High voltage operation – up to 200V

Scalable output power

Add more output transistors

Low distortion – 0.0005%

TI Information – Selective Disclosure
1818
Audio Power Amp Drivers
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
VBE =
0.7V
Nominal
VBias = 0.7V *
(RB2/(RP+RB1))
TI Information – Selective Disclosure
1919
Summary (Conclusion)
Circuit design Considerations
–Stability
–Thermal
•
PCB design considerations
–Grounding
•
Output Protection
–SPiKe
•
High Voltage Audio PA Driver (200V)
•
TI Information – Selective Disclosure
2020
Thank You
Click to edit Master subtitle style
TI Information – Selective Disclosure