STA7360

Block description

Doc ID 9814 Rev 4

19/28

4.7

Output stage

Poor current capability and low cutoff frequency are well-known limits of the standard lateral

PNP. Composite PNP-NPN power output stages have been widely used, regardless of their

high saturation drop. This drop can be overcome only at the expense of external

components, namely, the bootstrap capacitors. The availability of 4 A isolated collector PNP

(ICV PNP) adds versatility to the design. The performance of this component, in terms of

gain, VCEsat and cutoff frequency, is shown in Figure 27, 28, and 29 respectively. It is

realized in a new bipolar technology, characterized by top-bottom isolation techniques,

allowing the implementation of low leakage diodes, too. It guarantees BVCEO > 20 V and

BVCBO > 50 V both for NPN and PNP transistors. Basically, the connection shown in

Figure 30 has been chosen. First of all because its voltage swing is rail-to-rail, limited only

by the VCEsat of the output transistors, which are in the range of 0.3 W each. Then, the gain

VOUT/VIN is greater than unity, approximately 1+R2/R1. (VCC/2 is fixed by an auxiliary

amplifier common to both channel). It is possible, controlling the amount of this local

feedback, to force the loop gain (A * b) to less than unity at frequencies for which the phase

shift is 180°. This means that the output buffer is intrinsically stable and not prone to

oscillation.

Figure 27.

Figure 28.

Figure 29.