9

Introduction

In consumer electronics, an increasing demand has arisen for very high power monolithic audio amplifiers able to

match, with a low cost the performance obtained from the best discrete designs.

The task of realizing this linear integrated circuit in conventional bipolar technology is made extremely difficult by

the occurence of 2nd breakdown phenomenon. It limits the safe operating area (SOA) of the power devices, and

as a consequence, 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 protection 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 developed in a mixed bipolar-MOS high voltage technology called BCD

100.

9.1

Output stage

The main design task one is confronted with while developing an integrated circuit as a power operational

amplifier, independently of the technology used, is that of realizing the output stage.

The solution shown as a principle shematic by Figure 17 represents the DMOS unity-gain output buffer of the

TDA7294.

This large-signal, high-power buffer must be capable of handling extremely high current and voltage levels while

maintaining acceptably low harmonic distortion and good behaviour over frequency response; moreover, an

accurate control of quiescent current is required.

A local linearizing feedback, provided by differential amplifier A, is used to fullfil the above requirements, allowing

a simple and effective quiescent current setting.

Proper biasing of the power output transistors alone is however not enough to guarantee the absence of

crossover distortion.

While a linearization of the DC transfer characteristic of the stage is obtained, the dynamic behaviour of the

system must be taken into account.

A significant aid in keeping the distortion contributed by the final stage as low as possible is provided by the

compensation scheme, which exploits the direct connection of the Miller capacitor at the amplifier’s output to

introduce a local AC feedback path enclosing the output stage itself.

9.2

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 conditions.

Due to the absence of the 2nd breakdown phenomenon, the SOA of the power DMOS transistors 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 implemented in this device

combines a conventional SOA protection circuit with a novel local temperature sensing technique which "

dynamically" controls the maximum dissipation.

TDA7294

Introduction

DS0013 - Rev 8

page 13/31