STEP-DOWN, BUILT-IN FET, SYNCHRONOUS RECTIFICATION, PWM CONTROL SWITCHING REGULATORS

S-8550 Series

8

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Operation

1. Synchronous rectification PWM control step-down switching regulator

1. 1 Synchronous rectification

The synchronous rectification method lowers voltage drop to greatly reduce power dissipation since an Nch

power MOS FET, having resistance much lower than conventional switching regulators, is used.

In conventional switching regulators, current flows in the diode connected between the GND and CONT pins

to 0.7 V, so the power dissipation used to be very large. Synchronous rectification ultra-low resistance Nch

transistors repeat on and off, in synchronization with the operation of the Pch driver, in the reverse cycle of

the Pch driver. Moreover, the built-in P and N through prevention circuit helps much reduction of power

consumption during operation.

1. 2 PWM control

The S-8550 Series is a switching regulator using a pulse width modulation method (PWM) and features low

current consumption.

In conventional PFM control switching regulators, pulses are skipped when the output load current is low,

causing a fluctuation in the ripple frequency of the output voltage, resulting in an increase in the ripple

voltage.

In the S-8550 Series, the switching frequency does not change, although the pulse width changes from 0% to

100% corresponding to each load current.

The ripple voltage generated from switching can thus be

removed easily using a filter because the switching frequency is constant.

2. Soft-start function

The soft-start circuit built in the S-8550 Series controls the rush current and the overshoot of the output voltage

when powering on, the ON/OFF pin is switched from the “L” level to the “H” level, or the UVLO operation is

released. A reference voltage adjustment method is adopted as the soft-start method.

3. Shutdown pin

This pin stops or starts step-up operations.

Switching the shutdown pin to the “L” level stops operation of all the internal circuits and reduces the current

consumption significantly. DO NOT use the shutdown pin in a floating state because it is not pulled up or

pulled down internally. DO NOT apply voltage of between 0.3 V and 0.9 V to the shutdown pin because

applying such a voltage increases the current consumption. If the shutdown pin is not used, connect it to the

VIN pin.

Table 6

Shutdown Pin

CR Oscillation Circuit

Output Voltage

“H”

Operates

Set value

“L”

Stops

Hi-Z

VIN

ON/OFF

VSS

Figure 8