LM4780 Dual 60W Audio Power Amplifier

LM4780


LM4780 Overview

Basic Features

The LM4780 stereo audio power amplifier chip was announced by National Semiconductor in the March 2004 news release Three New Audio Amplifiers For High-Quality Home Stereo Systems LM4780/81/82.

This amplifier chip packs a big punch in a small package! It is capable of delivering 60 Watts RMS per channel into 8 Ohms for < 0.1% THD, or 0.03% THD at lower levels. It also has low noise (114 dB at 50 W), high power supply rejection ratio (120 dB), high gain bandwidth product (8 MHz) and high slew rate (19 V/uS).

The package is only 29.2mm x 15.7mm (1.15" x 0.618") in size. The two differential-input high-gain amplifiers are on two separate silicon dice and have independent pin connections. The die are exactly the same as the one used in the LM3886 amplifier chip.

The LM4780 requires only a hand-full of external components to configure a DC-blocking input filter, a gain-setting feeback network, and a Zobel filter on the output to provide a load at high frequencies. It has the SPiKe (tm) overload protection circuitry and it can be easily bridged for 120 Watts or paralleled for low impedance loads (less than 4 Ohms).

The 24 page Data Sheet is available as a PDF (size 1031K) in the product folder. It includes schematics for several applications including bridge and parallel configurations and an in-depth discussion on design considerations for using the device.

LM4780 is a member of National Semiconductor's Overture (tm) series of audio power amplifier chips. The Audio Application Information page contains the following relevant Application Notes:

The LM4780 has generated quite a bit of interest on the Chip Amps forum of DiyAudio.com at the beginning of 2004, particularly a PCB design for a bridged amplifier (120 Watts mono).

Predecessors

The LM4780 is part of the latest generation in a long line of audio power amplifier chips from National Semiconductor. In particular, the line includes the LM1875, LM3875 and LM3886 chips which have been released since the early 1990's. There are also other related Overture chips with and without mute options, standby, dual amplifer packages and isolated cases.

The LM1875 and LM3875 are now classed by National Semiconductor as Legacy Amplifiers and will probably be taken out of production within a year or two. The November 1995 News Release for the isolated package versions of the LM1876 / 2876 / 3875 / 3876 / 3886 / 4700 is still available.

GainCard and GainClone

The trio of LM1875 / 3875 / 3886 chips are commonly used in the GainClone style of DIY power amplifiers, which are attempted copies or derivatives of the commercial GainCard audio power amplifier made by 47 Laboratories.

The GainCard was originally released in mid 1999 with an output power of 25 Watts per channel and was assumed to use a pair of LM1875 chips. A 50 Watt per channel GainCard was released in April 2000 and was assumed to use a pair of LM3875 chips. There are many reviews of the amplifier at 47 Laboratories site (scroll down to 4706 GainCard) and a review at the TNT audio - online HiFi review site. Mick Feuerbacher has done an analysis of purported photographs of the internals of the GainCard amplifier.


LM4780 Pin Connections

© National Semiconductor Corporation 2003. Reproduced for the purpose of fair research from the LM4780 PDF data sheet dated July 2003.

LM4780 Annotated


Click for a larger image (37K)

Click for a larger image (65K)
Each line represents a bond wire.

Annotated for the purpose of fair research by Glenn Baddeley, 24 Sep 2004, from the LM4780 PDF data sheet dated July 2003 and the LM3886 Die Data Sheet (Requires sign-on) dated 19 March 2002. Refer to the Design Log for an explanation of the annotations. Original diagrams © National Semiconductor Corporation 2003.

LM4780 Annotated Equivalent Schematic


Desiderata* for LM4780 Amplifier Designers

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

My private research has revealed several interesting facts. This information is not contained in any information published by National Semiconductor. The tips could be very useful for the designer of an amplifier which uses the LM4780 chip, but I cannot take responsibility for its accuracy or for any fault or problem in any design which utilises this knowledge in any way.

Glenn

*

The "NC" pins are truely "No Connection"

Pins 1, 3, 6, 13, 23, 24 and 26 are designated as "NC" or "No Connection". In existing published designs and the reference design in the National Semiconductor data sheet, these pins are variously connected to V+, GND and nothing. There is actually no particular electronic design requirement for them to be connected to anything at all, as the pins are independently mounted in the insulated packaging and have no internal connection.

  • They may be safely connected to any other pin or left unconnected.
     
  • As a design aid, they can be connected to adjacent pins to make PCB tracks wider for the power supply rails, or used as a "via" to carry PCB tracks from one row of pins through the second row of pins to other components.
Reference
*

The "GND" pins should be not be connected to the "Clean Earth"

Pins 12 and 19 are designated as "GND" but their function is not to set a common reference point for audio signals. Operational amplifiers with differential inputs, like the LM4780, do not require a common reference point. Establishment of a common for the input and output potentials is external to the amplifier and is usually set to the mid-point of the two 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 two "GND" pins of the LM4780 are used as internal reference points for constant current sources and other signals. They draw approximately 3 milliAmps each toward the V- power supply rail. They may also carry modulation currents generated by internal amplification of the audio signal.

  • The "GND" pins should be directly connected to the "star earth" point. Other connections to the "star earth" are the central 0V point of power supply, the speaker output common, the Zobel network, the power supply bypass capacitors, and the low level "clean earth" point which is used for the input signal and feedback network. The "clean earth" point is isolated by a low value resistor from the large currents which circulate through the "star earth" point.
     
  • The "GND" pins should not be connected into the path of the "clean earth" as it will produce unwanted DC and AC interaction with this particularly sensitive part of the circuit.

A portion of the schematic for a LM4780 amplifier illustrates the idea. It is based on a design presented in the LM4780 data sheet:

These recommendations are in conflict with the reference board designed by National Semiconductor. They offer no reasoning for their choice of connecting the "GND" pins to the "clean earth". Maybe they consider the effects to be insignificant in their design.

*

LM4780 is a dual LM3886

The LM4780 package contains two LMP3886AT silicon dice. This is the same die as used in the LM3886.

LM3886 design procedure and schematics may be applied to the LM4780, with a slight derating of 0.5 dB in the nominal output power, from 68 Watts down to 60 Watts per channel.

Reference


LM4780 Links

These links are referred to by these web pages:


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