LM7372

www.ti.com

SNOS926F – MAY 1999 – REVISED SEPTEMBER 2014

Application Details (continued)

For a Class B output stage, one transistor of the output pair will conduct the load current as the output voltage

swings positive, with the other transistor drawing no current, and hence dissipating no power. During the other

half of the signal swing, this situation is reversed, with the lower transistor sinking the load current and the upper

transistor cut off. The current in each transistor will be a half wave rectified version of the total load current.

Ideally neither transistor will dissipate power when there is no signal swing, but will dissipate increasing power as

the output current increases. However, as the signal voltage across the load increases with load current, the

voltage across the output transistor (which is the difference voltage between the supply voltage and the

instantaneous voltage across the load) will decrease and a point will be reached where the dissipation in the

transistor will begin to decrease again. If the signal is driven into a square wave, ideally the transistor dissipation

will fall to zero.

L

where

•

•

(2)

For the package, the power dissipation will be doubled since there are two amplifiers in the package, each

contributing half the swing across the load.

The circuit in Single Supply Application, Figure 24, is using the LM7372 as the upstream driver in an ADSL

application with Discrete MultiTone modulation. With DMT the upstream signal is spread into 32 adjacent

channels each 4 kHz wide. For transmission over POTS, the regular telephone service, this upstream signal from

the CPE (Customer Premise Equipment) occupies a frequency band from around 20 kHz up to a maximum

frequency of 135 kHz. At first sight, these relatively low transmission frequencies certainly do not seem to require

the use of very high speed amplifiers with GBW products in the range of hundreds of megahertz. However, the

close spacing of multiple channels places stringent requirements on the linearity of the amplifier, since non-

linearities in the presence of multiple tones will cause harmonic products to be generated that can easily interfere

with the higher frequency down stream signals also present on the line. The need to deliver 3rd Harmonic

distortion terms lower than

−75 dBc is the reason for the LM7372 quiescent current levels. Each amplifier is

running over 3mA in the output stage alone in order to minimize crossover distortion. The xDSL signal levels are

adjusted to provide a given power level on the line, and in the case of ADSL, this is an average power of 13

dBm. For a line with a characteristic impedance of 100

transformer shown in Figure 24 is part of a transceiver circuit, two back-termination resistors are connected in

is required to deliver 20 mW to this load.

(3)

Using Equation 2 with this value for signal swing and a 24V supply, the internal power dissipation per amplifier is

132.8mW. Adding the quiescent power dissipation to the amplifier dissipation gives the total package internal

power dissipation as

(4)

This result is actually quite pessimistic because it assumes that the dissipation as a result of load current is

simply added to the dissipation as a result of quiescent current. This is not correct, since the AB bias current in

the output stage is diverted to load current as the signal swing amplitude increases from zero. In fact with load

currents in excess of 3.3 mA, all the bias current is flowing in the load, consequently reducing the quiescent

component of power dissipation. Also, it assumes a sine wave signal waveform when the actual waveform is

composed of many tones of different phases and amplitudes which may demonstrate lower average power

dissipation levels.

Copyright © 1999–2014, Texas Instruments Incorporated

Submit Documentation Feedback

15

Product Folder Links: LM7372