7

General information

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 occurrence 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 80.

7.1

Output stage

The main design task to cope with, while developing an integrated circuit as a power operational amplifier,

regardless the technology used, is to develop the output stage. The solution shown as a principle schematic by

Fig 18 represents the DMOS unity-gain output buffer of the TDA7296.

This large-signal, high-power buffer must be able to handle 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 linearising feedback, provided by differential amplifier A, is used to fulfil 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 linearisation 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.

7.2

Protections

When a power IC is being designed, 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.

TDA7296

General information

DS0179 - Rev 11

page 11/23