TI Information – Selective Disclosure Click to edit Master subtitle style Audio Amplifier Design 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 efficiency  Smaller size Why Class A/B? LM4780 Stereo A/B 60W Stereo Class D 50W

TI Information – Selective Disclosure 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. Parallel Operation

TI Information – Selective Disclosure 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 Bridge Operation 44

TI Information – Selective Disclosure 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.  Snubber  A simple R/C filter on the output will usually fix a bottom side oscillation  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. Stability What to look for 55 Snubber Av > 10 Bypass Caps

TI Information – Selective Disclosure Power Dissipation Thermal Resistance  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  Determined by the path the heat takes to get “out” of the package   ja is referred to as “junction-to-ambient”   jc is referred to as “junction-to-case”  Heat-sinks also have a thermal resistance specified in Degrees C/W. Thermal Considerations What factors are involved 66

TI Information – Selective Disclosure Determine the operating conditions Use the PDMax equation  LM1875 datasheet  Supply voltage = +/- 25V  Load = 8 ohms  PDMax = V2(supply total)/(2*  2RLoad) + PQ Thermal Considerations How to calculate power dissipation – LM Calculate PDMax for LM875  PDMax = (50v)/(2*(3.14)2*8) + (50v*70Ma)  PDMax = = 18.85W

TI Information – Selective Disclosure 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 Considerations Power Dissipation Curves 88

TI Information – Selective Disclosure Thermal Resistance LM1875  The total thermal resistance must be calculated   LM1875  jc + Heat Sink Thermal Resistance) = (3oC/W + 2oC/W) - assume heat-sink of 2oC/W = 5oC/W Thermal Considerations How 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

TI Information – Selective Disclosure 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 Thermal Considerations Power de-rating Curves 1010

TI Information – Selective Disclosure Current Flow 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. PCB Layout Ground Trace Routing 1111  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

TI Information – Selective Disclosure Setup Analysis  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  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 PCB Layout How to Evaluate 1212

TI Information – Selective Disclosure The Fix  The input ground is now disconnected from the output ground.  The ground is routed to the quiet ground (Cap Ground) PCB Layout Ground Trace Routing 1313

TI Information – Selective Disclosure 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

TI Information – Selective Disclosure Audio Power Amplifier Roadmap LM3886 1x70W Overture LM3875,76 1x60W Overture LM2876 1x70W Overture LM4780 2x60W Overture LM4781,2 3x35W Overture LM4702 2x125W Driver LM1875 1x30W LM4752,55 2x11W LM4950 2x3, 1x10W LME x400W Driver R el ati ve Pe rfo rm an ce 1515 Mid Power 0.02% THD Overture SPiKe Protection 0.002% THD LME Series % THD

TI Information – Selective Disclosure 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 Curre nt Flow V I

TI Information – Selective Disclosure Feature s Beneifits  Current Limiting  Overvoltage Protection  SPiKe Protection  Self Peak  Instantaneous  Temperature (Ke)  Built into the output transistors  Acts instantly  Monitors all portions of the output transistors Output Protection Overture SPiKeTM Protection 1717 Beneifits  Overture power amps do not fail

TI Information – Selective Disclosure 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 – % Audio Power Amp Drivers 1818

TI Information – Selective Disclosure 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 = 0.7V Nominal VBias = 0.7V * (RB2/(RP+RB1))

TI Information – Selective Disclosure 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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