Datasheet

LM4670
SNAS240C –DECEMBER 2004–REVISED MAY 2013 www.ti.com
APPLICATION INFORMATION
GENERAL AMPLIFIER FUNCTION
The output signals generated by the LM4670 consist of two, BTL connected, output signals that pulse
momentarily from near ground potential to V
DD
. The two outputs can pulse independently with the exception that
they both may never pulse simultaneously as this would result in zero volts across the BTL load. The minimum
width of each pulse is approximately 350ns. However, pulses on the same output can occur sequentially, in
which case they are concatenated and appear as a single wider pulse to achieve an effective 100% duty cycle.
This results in maximum audio output power for a given supply voltage and load impedance. The LM4670 can
achieve much higher efficiencies than class AB amplifiers while maintaining acceptable THD performance.
The short (350ns) drive pulses emitted at the LM4670 outputs means that good efficiency can be obtained with
minimal load inductance. The typical transducer load on an audio amplifier is quite reactive (inductive). For this
reason, the load can act as it's own filter, so to speak. This "filter-less" switching amplifier/transducer load
combination is much more attractive economically due to savings in board space and external component cost
by eliminating the need for a filter.
POWER DISSIPATION AND EFFICIENCY
In general terms, efficiency is considered to be the ratio of useful work output divided by the total energy required
to produce it with the difference being the power dissipated, typically, in the IC. The key here is “useful” work. For
audio systems, the energy delivered in the audible bands is considered useful including the distortion products of
the input signal. Sub-sonic (DC) and super-sonic components (>22kHz) are not useful. The difference between
the power flowing from the power supply and the audio band power being transduced is dissipated in the
LM4670 and in the transducer load. The amount of power dissipation in the LM4670 is very low. This is because
the ON resistance of the switches used to form the output waveforms is typically less than 0.25Ω. This leaves
only the transducer load as a potential "sink" for the small excess of input power over audio band output power.
The LM4670 dissipates only a fraction of the excess power requiring no additional PCB area or copper plane to
act as a heat sink.
DIFFERENTIAL AMPLIFIER EXPLANATION
As logic supply voltages continue to shrink, designers are increasingly turning to differential analog signal
handling to preserve signal to noise ratios with restricted voltage swing. The LM4670 is a fully differential
amplifier that features differential input and output stages. A differential amplifier amplifies the difference between
the two input signals. Traditional audio power amplifiers have typically offered only single-ended inputs resulting
in a 6dB reduction in signal to noise ratio relative to differential inputs. The LM4670 also offers the possibility of
DC input coupling which eliminates the two external AC coupling, DC blocking capacitors. The LM4670 can be
used, however, as a single ended input amplifier while still retaining it's fully differential benefits. In fact,
completely unrelated signals may be placed on the input pins. The LM4670 simply amplifies the difference
between the signals. A major benefit of a differential amplifier is the improved common mode rejection ratio
(CMRR) over single input amplifiers. The common-mode rejection characteristic of the differential amplifier
reduces sensitivity to ground offset related noise injection, especially important in high noise applications.
PCB LAYOUT CONSIDERATIONS
As output power increases, interconnect resistance (PCB traces and wires) between the amplifier, load and
power supply create a voltage drop. The voltage loss on the traces between the LM4670 and the load results is
lower output power and decreased efficiency. Higher trace resistance between the supply and the LM4670 has
the same effect as a poorly regulated supply, increase ripple on the supply line also reducing the peak output
power. The effects of residual trace resistance increases as output current increases due to higher output power,
decreased load impedance or both. To maintain the highest output voltage swing and corresponding peak output
power, the PCB traces that connect the output pins to the load and the supply pins to the power supply should
be as wide as possible to minimize trace resistance.
The use of power and ground planes will give the best THD+N performance. While reducing trace resistance, the
use of power planes also creates parasite capacitors that help to filter the power supply line.
10 Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated
Product Folder Links: LM4670