STEP-DOWN, SYNCHRONOUS PWM CONTROL SWITCHING REGULATOR CONTROLLER

S-8533 Series

9

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Operation

1. Synchronous PWM Control Step-down DC-DC Converter

1. 1 Synchronous Rectification

A synchronous rectifying DC-DC converter enables a greater reduction in the power consumption of the external

rectifying element compared with a conventional DC-DC converter. In addition, incorporating a P and N feed-

through prevention circuit reduces the feed-through current during operation of external transistors (P-channel and

N-channel), making the operating power consumption extremely low.

1. 2 PWM Control

The S-8533 Series is a DC-DC converter that uses pulse width modulation (PWM) and is characterized by its low

current consumption.

In conventional modulation PFM system DC-DC converters, pulses are skipped when they are operated with a low

output load current, causing variations in the ripple frequency of the output voltage and an increase in the ripple

voltage. Both of these effects constitute inherent drawbacks to those converters.

In the S-8533 Series, the pulse width varies in the range from 0 to 100% according to the load current, yet the ripple

voltage produced by the switching can easily be eliminated by a filter since the switching frequency is always

constant. When the pulse width is 0% (when there is no load or the input voltage is high), current consumption is

low since pulses are skipped.

2. Soft-Start Function

The S-8533 Series has a built-in soft-start circuit.

overshooting of the output voltage, when the power is switched on or the ON/OFF pin is set “H”.

The time needed for the output voltage to reach 95% of the set output voltage value is calculated by the following

formula.

0

10

20

30

40

50

60

0

5000

10000

15000

20000

Figure 7 Soft-Start Time

time. If the soft-start time is short, possibility for output voltage overshoot, input current rush, and malfunction of the

IC increases.