STEP-DOWN, SYNCHRONOUS PWM CONTROL SWITCHING REGULATOR CONTROLLER
S-8533 Series
Rev.2.3_00
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.
This circuit enables the output voltage (VOUT) to rise gradually over the specified soft-start time (tSS) to suppress the
overshooting of the output voltage, when the power is switched on or the ON/OFF pin is set “H”.
The soft-start time can be set with an external capacitance (CSS).
The time needed for the output voltage to reach 95% of the set output voltage value is calculated by the following
formula.
tSS [ms] = 0.002 × CSS [pF]
60
50
40
30
20
10
0
0
5000
10000
15000
20000
External capacitance (CSS) [pF]
Figure 7 Soft-Start Time
The value for CSS should be selected to give enough margin to the soft-start time against the power supply rise
time. If the soft-start time is short, possibility for output voltage overshoot, input current rush, and malfunction of the
IC increases.
8
Seiko Instruments Inc.
SII [ SEIKO INSTRUMENTS INC ]
