MB3759 Datasheet, PDF (11/27 Pages) Fujitsu Component Limited. – Switching Regulator Controller (Switchable between push-pull and single-end functions)

MB3759
s BASIC OPERATION
Switching regulators can achieve a high level of efficiency. This section describes the basic principles of operation
using a chopper regulator as an example.
As shown in the diagram, diode D provides a current path for the current through inductance L when Q is off.
Transistor Q performs switching and is operated at a frequency that provides a stable output. As the switching
element is saturated when Q is on and cutoff when Q is off, the losses in the switching element are much less
than for a series regulator in which the pass transistor is always in the active state.
While Q is conducting, the input voltage VIN is supplied to the LC circuit and when Q is off, the energy stored in
L is supplied to the load via diode D. The LC circuit smooths the input to supply the output voltage.
The output voltage VO is given by the following equation.
VO = ^Ton VIN = ^Ton VIN
Ton + Toff
T
Q : ON
L
Q
VIN
D
Q : OFF
C ^VO
RL
Q: Switching element
D: Flywheel diode
As indicated by the equation, variation in the input voltage is compensated for by controlling the duty cycle (Ton/
T). If VIN drops, the control circuit operates to increase the duty cycle so as to keep the output voltage constant.
The current through L flows from the input to the output when Q is on and through D when Q is off. Accordingly,
the average input current IIN is the product of the output current and the duty cycle for Q.
IIN =
Ton _IO
T
The theoretical conversion efficiency if the switching loss in Q and loss in D are ignored is as follows.
η=
PO
PIN
× 100 (%)
=
VO ^· IO
VIN ^· IIN
× 100
=
VIN ^· IO ^· Ton / T
VIN ^· IO ^· Ton / T
= 100 (%)
× 100
The theoretical conversion efficiency is 100%. In practice, losses occur in the switching element and elsewhere,
and design decisions to minimize these losses include making the switching frequency as low as practical and
setting an optimum ratio of input to output voltage.
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