INTRODUCTION

In consumer electronics, an increasing demand

has arisen for very high power monolithic audio

amplifiers able to match, with a low cost the per-

formance obtained from the best discrete de-

signs.

The task of realizing this linear integrated circuit

in conventional bipolar technology is made ex-

tremely difficult by the occurence of 2nd break-

down phenomenon. It limits the safe operating

area (SOA) of the power devices, and as a con-

sequence, the maximum attainable output power,

especially in presence of highly reactive loads.

Moreover, full exploitation of the SOA translates

into a substantial increase in circuit and layout

complexity due to the need for sophisticated pro-

tection circuits.

To overcome these substantial drawbacks, the

use of power MOS devices, which are immune

from secondary breakdown is highly desirable.

The device described has therefore been devel-

oped in a mixed bipolar-MOS high voltage tech-

nology called BCD 100.

1) Output Stage

The main design task one is confronted with while

developing an integrated circuit as a power op-

erational amplifier, independently of the technol-

ogy used, is that of realizing the output stage.

The solution shown as a principle shematic by Fig

15 represents the DMOS unity-gain output buffer

of the TDA7294.

This large-signal, high-power buffer must be ca-

pable of handling extremely high current and volt-

age levels while maintaining acceptably low har-

monic distortion and good behaviour over fre-

quency response; moreover, an accurate control

of quiescent current is required.

A local linearizing feedback, provided by differen-

tial amplifier A, is used to fullfil the above require-

ments, allowing a simple and effective quiescent

current setting.

Proper biasing of the power output transistors

alone is however not enough to guarantee the ab-

sence of crossover distortion.

While a linearization of the DC transfer charac-

teristic of the stage is obtained, the dynamic be-

haviour of the system must be taken into account.

A significant aid in keeping the distortion contrib-

uted by the final stage as low as possible is pro-

vided by the compensation scheme, which ex-

ploits the direct connection of the Miller capacitor

at the amplifier’s output to introduce a local AC

feedback path enclosing the output stage itself.

2) Protections

In designing a power IC, particular attention must

be reserved to the circuits devoted to protection

of the device from short circuit or overload condi-

tions.

Due to the absence of the 2nd breakdown phe-

nomenon, the SOA of the power DMOS transis-

tors is delimited only by a maximum dissipation

curve dependent on the duration of the applied

stimulus.

In order to fully exploit the capabilities of the

power transistors, the protection scheme imple-

mented in this device combines a conventional

SOA protection circuit with a novel local tempera-

ture sensing technique which ” dynamically” con-

trols the maximum dissipation.

Figure 15: Principle Schematic of a DMOS unity-gain buffer.

TDA7294

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