LME49810 Audio Power Amplifier Driver with Baker Clamp

  LME49810   LME49810


Overview

Basic Features

National Semiconductor introduced the LME49810 integrated circuit in May 2007. There was an official news release made on 23rd July 2007.

The LME49810 consists of a differential-input high-gain operational-amplifier, but there are several features which set it apart from most single op-amps. The LME49810 has a remarkable output swing of up to 190 Volts peak-to-peak, and the output is split into two separate halves, a 'source' and a 'sink'. This makes it ideal for driving the output stage of an audio power amplifier and in excess of 500 Watts RMS can be delivered into an 8 Ohm load.

There are two bias pins for connecting a Vbe multiplier for temperature compensation of the output stage quiescent conditions. The Baker Clamp feature provides soft clipping when the amplifier is overdriven by preventing the driver transistors from saturating. This reduces unwanted artefacts at high peak output power levels. A 'clip flag' pin allows the clipping condition to be indicated by a LED.

The 'source' and 'sink' outputs can pass up to 50 milliAmps of current. Depending on the output power, the output transistors (BJTs) may not need to be preceded by driver transistors. It is also possible to adapt the LME49810 to drive a variety of power semiconductor devices such as directly driving MOSFETs, or even thermionic valves (tubes). The outputs have a maximum permitted differential of 10 Volts.

Have you read something like this before? Maybe you are thinking of the LM4702 dual driver op-amp, for which I have my own web page.

In recent years the popularity of "chip amp" or "gainclone" audio power amplifiers has centred around the simplicity of using modern amplifier chips which have the power output stages integrated on to the chip. An advantage of the LME49810 is that output stage can be whatever the designer cares to use. This is discussed further on the LM4702 page.

Performance

The Total Harmonic Distortion plus Noise (THD+N) characteristic of the LME49810 is very good. At 30 Volts RMS output of a 1 KHz sine wave, THD+N is about 0.0007% across a 20 Hz to 30 KHz bandwidth.

Some other important specifications of the LME49810 are: Slew rate is 50 V/uS. Open loop voltage gain is 120 dB at DC. Power Supply Rejection Ratio (PSRR) is 110 dB. There is also a 'mute' function which can be used to start up or shut down the amplifier. It has automatic thermal shut-down at 150 °C but no other overload protection circuitry.

Product Data

The Data Sheet (PDF, size 867K, 22 pages) is available in the LME49810 product folder. It contains a schematic for a typical application (JPEG image, size 31K) and the PCB layout of the evaluation board. Information is also provided on the mute function, thermal protection, power dissipation, heat sinking, selection of external components and tips on avoiding thermal run-away when using bipolar output stages.

The part is available in one grade, which is a customised TO-247 package. It is in full production as at 1 June 2007. The approximate unit prices is not available as at 1 June 2007.

Practical Experiences

The LME49810 generated some interest on the Chip Amps forum of DiyAudio.com, commencing in a LM4702 discussion thread on 31st May 2007. I started a LME49810 discussion thread on 4th June 2007.

Previous Generation Drivers

The concept of a high output swing op-amp for driving an audio power output stage is not new, and this is discussed further on the LM4702 page.


Pin Connections

    

    
©
National Semiconductor Corporation 2007. Reproduced and annotated for the purpose of fair research from the LME49810 PDF data sheet dated May 2007.


Desiderata* for LME49810 Amplifier Designers

* Plural of Desideratum. Something considered necessary or highly desirable. Dictionary reference

My private research has revealed a few useful tips for the designer using this chip. I do not take any responsibility for their accuracy or for any fault or problem in any design which utilises this knowledge in any way. Glenn.


*  Preferred value for Mute Resistor

Page 13 of the LM49810 Data Sheet (PDF, size 867K, 22 pages) provides an equation to determine the mute current for a given mute resistor:

Imute = ( Vmute - 0.7 ) / ( Rmute + 10000 )

Vmute is the power supply voltage from which the Rmute resistor is connected to pin 2. The 0.7 Volt term represents the forward voltage drop of a Base-Emitter junction which is biased by the mute current. The 10,000 Ohm term represents a resistor which is internal to the chip.

The mute current should be about 50 uA to 100 uA to safely put the amplifier into 'play' mode, although it is in this mode for currents from about 20 uA up to the maximum specified 200 uA. See the graph in the upper right corner of page 11 of the data sheet.

Assuming a design mute current centred on 100 uA and an acceptable variation from 50 uA to 150 uA, an upper and lower range of resistance can be calculated for a given power supply. It's then a matter of choosing a preferred value resistor which fits within this range.

To save some calculations, the following graph shows the mute current (I mute) on the vertical axis and the mute supply voltage (Vmute) on the horizontal axis. The zones of current are colour coded. The central 'play' zone is coded green on the vertical axis, and the 'no-play' zone is red. Lines have been plotted through the 'play' zone for the E12 preferred resistance values and they are labelled in kilo Ohms.

The design procedure is to start with the mute power supply voltage and locate it on the horizontal axis. Take a vertical line up until it meets sloping lines in the green 'play' zone. Use the legend on the right side of the graph to find the preferred resistor value which represents the line. There is a worked example for the LM4702 chip available for further guidance, although the mute current range is quite different.

The power dissipation of the resistor is very low, a 1/4 Watt rated part can be used at all operating points. The graph was generated using an Excel spreadsheet (Excel file, size 50K).

16 October 2007


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