LM4702 Audio Power Amplifier Driver

  LM4702   LM4702


Overview

Basic Features

National Semiconductor introduced the LM4702 integrated circuit in about August 2005. The official news release was on 19th September 2006, heralding it as the first member of a new family of high performance audio amplifiers. In 2007 the new family grew to include the LME49810 (200V Audio Power Amplifier Driver with Baker Clamp) and the LME49710 (High Performance HiFi Audio Op Amp in single, dual LME49720 / LM4562 and quad LME49740 packages).

The LM4702 consists of two differential-input high-gain operational-amplifiers, but there are two features which set it apart from most dual op-amps. The LM4702C has a remarkable output swing of up to 144 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 stages of a stereo audio power amplifier. In excess of 300 Watts RMS per channel can be delivered into 8 Ohm loads.

The 'source' and 'sink' outputs can only pass about 5.5 milliAmps of current, so power output transistors (BJTs) must be preceded by driver transistors, such as Darlington pairs or Sziklai pairs (also known as Complementary Feedback Pairs). It is also possible to adapt the LM4702 to drive a variety of power semiconductor devices such as MOSFETs, or even thermionic valves (tubes). The outputs have a maximum permitted differential of 6 Volts.

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. These can sound very good when combined with a short feedback path and quality passive components. Some of the most commonly used "chip amp" devices from National Semiconductor are the LM1875, LM3875, LM3886 and LM4780. The specifications of these chips are comparable, and the LM4780 is a dual 60W RMS part. A disadvantage is that the topology and operating conditions of the power output stage is "carved in silicon" and cannot be changed.

The LM4702 liberates the output stage to be whatever the designer cares to use. There are literally dozens of possibilities which have their own advantages, disadvantages and sonic signatures. This can be a great opportunity for a competent electronic designer, but a confusing situation for the average enthusiast. The emergence of practical cook-book designs for output stages and PCBs will simplify construction for the mainstream audio DIY-er, such as the amplifiers from TECHDIY.COM.

Performance

The Total Harmonic Distortion plus Noise (THD+N) characteristic of the LM4702C is quite good. At 10 Volts RMS output, THD+N is about 0.005% across a 20 Hz to 20 KHz bandwidth. It steadily drops to about 0.0015% at maximum output voltage. At very low output voltages the data sheet shows that THD+N rises to over 0.1%, but this may be due to the design of the test circuit or the layout of the evaluation board contributing extra noise and induced distortion.

The LM4702 product folder lists Application Note AN-1490 LM4702 Power Amplifier (PDF, size 1034K, 10 pages, published May 2006). It presents a 100 Watt RMS stereo power amplifier design using the LM4702C which achieves less than 0.002% THD+N at 1 Watt RMS output, falling to 0.0006% at the highest power before clipping. The THD+N rises to a maximum of 0.02% at a miniscule output of 10 milliWatts RMS. The schematic (JPEG image, 57K size) shows the output stage uses a single pair of complementary darlington transistors per channel. Particular attention was given to achieving the lowest practical THD+N by optimising the PCB design, careful selection of components and close attention paid to construction and wiring layout. The two toroidal mains transformers required mu-metal shields for the best THD+N performance.

Another Application Note AN-1645 LM4702 Driving a MOSFET Output Stage (PDF, size 656K, 24 pages, published May 2007) covers the design of a 125 Watt RMS stereo power amplifier with the LM4702 directly driving a pair of power MOSFETS. The choice of MOSFET is discussed in detail and biasing versus distortion is optimised for MOSFET devices from various manufacturers. A complete schematic is presented.

Some other important specifications of the LM4702C are: Slew rate is 15 V/uS. Open loop voltage gain is 93 dB. Cross-talk is 85 dB. 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 and Grades

The Data Sheet (PDF, size 891K, 16 pages) is available in the LM4702 product folder. It contains a schematic for a typical application (JPEG image, size 36K) 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 classified into 3 grades which have different maximum power supply voltages and hence different output swing capabilities:

Part numberPackageMaximum power supply range (Volts)CommentApproximate unit price (USD$)
LM4702CTO-220, 15 pin+/- 85Full production in mid 2006.5 - 10
LM4702BTO-220, 15 pin+/- 100 The production specifications were published in August 2006. Full production started in early 2007.25 - 35
LM4702AMilitary specifcation gold plated TO-3, 15 pin+/- 100 (tentative) Pre-release specifications were first published in August 2006. This part is still in development and final specifications have not been published as at May 2007.150 - 200

Practical Experiences

The LM4702 generated some interest on the Chip Amps forum of DiyAudio.com commencing in October 2005. Discussion continued through 2006 and into 2007, including presentation of some schematics and PCB designs using the part. Links are provided to the main discussion threads.

TECH-DIY.COM has several DIY projects, including two amplifiers which use LM4702:

BACKDOOR has a prototype printed cicuit board for a LM4702 stereo audio power amplifier.

The Audio Knowledge Base at National Semiconductor added 3 items on the LM4702 in mid 2006, and they are still current at May 2007:

  1. How do I get an output power of 300W or more per channel for the LM4702?
    The output stage will need to be composed of 3 NPN/PNP Output Pairs. It is also possible to get 100W/Ch with 1 output pair, and 100W-200W/CH with 2 NPN/PNP output pairs.
     
  2. Do I need a heat sink for the LM4702?
    You do not need a heat sink on the LM4702 itself. However, it is necessary to use a heat sink on the output stage. The LM4702 does have internal thermal protection, but this does not monitor the output stage.
     
  3. Why do I need an output stage for the LM4702?
    The LM4702 both conditions the input signal and produces a very high output voltage swing. However, the output current is very small at 5.5mA typical. This means that it needs output (current driving) transistors to produce the current needed to supply high power to 3, 4 and 8 Ohm loads.
     

Previous Generation Drivers

The concept of a high output swing op-amp for driving an audio power output stage is not new. The TDA7250 "60 Watt Hi-Fi dual audio driver" has been around since the mid 1990's. The Data Sheet (PDF, size 138K, 11 pages) contains an application circuit (JPEG image, size 42K) which uses the low cost TIP142/147 power darlington transistors. There is a kit available from AmpsLAB which uses this 20 pin Dual-In-Line IC.

Back in the mid 1970's, National Semiconductor's own LM143 "High Voltage Operational Amplifier" could be used to drive the output stage of a single channel 90 Watt audio power amplifer. The Data Sheet (PDF, size 229K, 12 pages) and Application Note AN-127: LM143 Monolithic High Voltage Operational Amplifier Applications (PDF, 249K, 12 pages) of April 1976, which contains an amplifier schematic (JPEG image, size 48K).

Another example, from the 1990s, is LM391 "Audio Power Driver". It could be used to drive external transistors to deliver 10 to 100 Watts of power at 0.01% THD. The Data Sheet (PDF, size 237K, 12 pages) contains typical application designs for 20 Watt (JPEG image, size 36KB) and 40 Watt (JPEG image, size 45KB) amplifiers. Both LM143 and LM391 were taken off the market in 1998.

The LM4702 uses a new fabrication process and modern design which results in superior performance in all respects and requires much simpler ancillary circuitry than the TDA7250, LM143 or LM391.


Pin Connections

    
©
National Semiconductor Corporation 2005. Reproduced and annotated for the purpose of fair research from the LM4702 PDF data sheet dated November 2005.


Desiderata* for LM4702 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 10 of the LM4702 Data Sheet (PDF, size 825K, 15 pages) provides an equation to determine the mute current for a given mute resistor:

Imute = ( Vcc - 2.9 ) / Rmute

The mute current must be at least 1 mA to put the amplifier into 'play' mode, but any more than 2 mA will exceed the specified maximum current. Assuming a design mute current centred on 1.5 mA and an acceptable variation from 1.25 mA to 1.75 mA, 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 positive supply voltage (Vcc) on the horizontal axis. The zones of current are colour coded. The central 'play' range is green, and the marginal 'play' range is purple. The over-current 'no go' and 'mute' ranges are red. Lines have been plotted through the 'play' range for the E12 preferred resistance values and they are labelled in kilo Ohms.

The design procedure is to start with the peak positive 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. Follow the sloping lines up to the right to find the preferred resistor values. See the worked example for further guidance.

The power dissipation of the resistor is quite low, a 1/4 Watt rated part can be used at all operating points. Even at Vcc = 80 V and Imute = 1.75 mA the dissipation is only 0.144 W. A 1/8 Watt rated part can be used up to about Vcc = 60 V, where the dissipation is 0.1 W at Imute = 1.75 mA.

The graph was generated using an Excel spreadsheet (Excel file, size 22K).

4 June 2006


*  Heatsinking Requirements

The LM4702 only needs to dissipate about 1 Watt when the +Vcc and -Vee power supply rails are 20 Volts each. Under these minimum power supply conditions and without a heatsink, the temperature of the metal tab sits at about 22 °C above ambient. When the rails are increased to their 75 Volt maximum the dissipation is about 4.1 Watts. The metal tab becomes very hot, as much as 90 °C above ambient. The chip continues to function because the junction temperature is still within its operating range of 150 °C maximum. This is probably why National Semiconductor's Audio Knowledge Base states "You do not need a heat sink on the LM4702 itself".

However, for reliable long-term operation the temperature rise should be no more than about 50 °C above ambient. If you can't hold your finger on a component for at least 2 seconds it is probably running hotter than this recommended limit.

An easy heatsinking solution for the LM4702 is to bolt the metal tab to the heatsink used by the power output devices. The metal tab is internally connected to the -Vee power supply rail, so make sure the tab is electrically isolated from the heatsink using an insulating washer smeared with thermal grease. Ensure the mounting bolt or clamp also provides electrical isolation.

The following graph shows the measured temperature rise above ambient for a typical range of power supply voltages. The upper two plot lines are for no heatsink, with the LM4702 mounted above the PCB (as it normally would be) and mounted below the PCB (which has reduced cooling by air convection). Beyond about 45 Volts the chip begins to get too hot.

  

The lower two plot lines are for a small aluminium TO-220 heatsink bolted to the LM4702 tab with no insulating washer. The Thermalloy 6106-B 13 heatsink was used, which is approximately 30 X 25 X 13 mm in size and has a thermal resistance of about 17 °C/Watt. Even this very modest heatsink makes a big difference to the temperature rise. At power supply rails of 50 Volts it is just warm to the touch. At the maximum supply rails it is still within the recommended temperature rise for reliable operation.

See the Design Log for photos and a description of the test rig. The graph was generated using an Excel spreadsheet (Excel file, size 21K).

20 June 2006


*  Ground pin should be not be connected to Clean Earth

Pin 7 is designated as GND but its function is not to set a common ground point for audio signals. Operational amplifiers with differential inputs, like the LM4702, do not require such an internal reference point. Establishment of a reference point for the input and output potentials is external to the amplifier and is usually set to the centre of the power supply rails by biasing the differential inputs via resistive paths and providing a DC feedback path from the output. This is so that maximum voltage swing can be achieved on the output and to simplify power supply design. The amplifier internals are not actually aware, or particularly care, about how this common point is set, or its voltage in relation to the power supplies. It just amplifies the voltage difference between the inputs by the open loop gain.

The GND pin on the LM4702 is used as an internal reference point for constant current sources and other signals. It draws between 1.7 and 1.8 milliAmps towards the negative Vee power supply rail. Within this range the standing current has a non-linear relationship with the power supply voltage and the mute current (JPEG image, size 29K). When amplifying a signal to the maximum output current and voltage capability the GND pin also carries a 0.3 milliAmp peak-to-peak modulation current which is distorted version of the output voltage; the peaks of the waveform are noisy and flattened.

A portion of a schematic for a LM4702 amplifier illustrates the idea:

13 July 2006


Links

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