AN557 datasheet(2/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

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Figure 1. BCD process and increase in power ICs complexity.

Figure 2. Cross Section of the BCD Mixed Technology.

THE STEP-DOWN CONFIGURATION

Fig. 3 shows the simplified block diagram of the circuit realizing the step-down configuration. This circuit

operates as follows : Q1 acts as a switch at the frequency f and the ON and OFF times are suitably con-

trolled by the pulse width modulator circuit. When Q1 is saturated, energy is absorbed from the input which

is transferred to the output through L. The emitter voltage of Q1, VE, is Vi-Vsat when Q is ON and -VF (with

VF the forward voltage across the D diode as indicated) when Q1 is OFF. During this second phase the

current circulates again through L and D. Consequently a rectangular shaped voltage appears on the emit-

ter of Q1 and this is then filtered by the L-C-D network and converted into a continuous mean value across

the capacitor C and therefore across the load. The current through L consists of a continuous component,

ILOAD, and a triangular-shaped component super-imposed on it, ∆IL, due to the voltage across L.

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