SA57254-30GW Datasheet, PDF (10/18 Pages) NXP Semiconductors – CMOS switching regulator (PWM controlled)

Philips Semiconductors
CMOS switching regulator (PWM controlled)
Product data
SA57254-XX
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.
V_IN
C_IN
L₀
SW
PASSIVE SNUBBER
(OPTIONAL)
D
V_OUT
V_DD
FB
C_OUT
SA57254-XX
GND
Figure 24. Boost converter.
SL01504
SPIKE
ENERGY BEING
STORED IN INDUCTOR
+V_IN
PERIOD 1
ENERGY BEING TRANSFERRED TO OUTPUT
+V_OUT
CORE EMPTY,
PARASITIC CIRCULATING ENERGY
PERIOD 2
PERIOD 3
0
V_IN
L₀
I_SW × R_DS(ON)
I_peak
(V_IN – V_OUT)
L₀
Figure 25. Boost converter waveforms (discontinuous mode).
SL01464
2003 Nov 11
10