TDA7293

Circuit description

Doc ID 6744 Rev 8

7/21

3

Circuit description

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 phoenomenon. 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 of 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 BCDII 100/120.

3.1

Output Stage

The main design task in developping a power operational amplifier, independently of the

technology used, is that of realization of the output stage.

The solution shown as a principle shematic by Fig6 represents the DMOS unity - gain output

buffer of the TDA7293.

Figure 4.

Schematic of a DMOS unity-gain buffer

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.