Philips Semiconductors

Product data

SA57254-XX

CMOS switching regulator (PWM controlled)

2003 Nov 11

10

APPLICATION INFORMATION

The SA57254-XX can be used for a simple boost (step-up)

converter or the less commonly used flyback converter (isolated

boost). The major operating restriction of the simple boost converter

is that its output voltage must always be above the highest

expected value of the input voltage. The flyback converter circuit

requires more parts, but the output voltage is not restricted by the

input voltage.

Boost converter fundamentals

The boost or step-up converter is a non-dielectrically isolated

switching power supply topology (arrangement of power parts). That

is, the input power source is directly connected to the output load

(ground and signals). A typical boost converter, with an optional

passive snubber, can be seen in Figure 24.

To understand the boost converter’s operation, examine its three

periods of operation. These periods are: the power switch on-time

(period 1); the inductor discharge period (period 2); and the inductor

empty state (period 3). These periods and their associated currents

can be seen in Figure 25.

SL01504

D

GND

SA57254-XX

SW

PASSIVE SNUBBER

(OPTIONAL)

FB

Figure 24. Boost converter.

SL01464

0

CORE EMPTY,

PARASITIC CIRCULATING ENERGY

ENERGY BEING TRANSFERRED TO OUTPUT

SPIKE

ENERGY BEING

STORED IN INDUCTOR

PERIOD 1

PERIOD 2

PERIOD 3

Figure 25. Boost converter waveforms (discontinuous mode).