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SS6550

Introduction

The SS6550 is a low-noise, pulse-width-modulated

(PWM), DC/DC step-down converter. It features an in-

ternal synchronous rectifier, which eliminates the ex-

ternal Schottky diode. The SS6550 is suitable for

Li-lon battery applications, or can be used with 3V or

5V fixed input voltages. It operates in one of the follow-

ing four modes

1.

The SS6550 can operate in PWM mode

with a fixed frequency, regardless of its

load.

2.

In synchronizable PWM mode, it allows an

external switching frequency to control and

minimize harmonics.

3.

In idle mode (PWM/PFM), it can extend

battery life by switching to PFM pulse-

skipping mode during light loads.

4.

In shutdown mode, the device will stop

working and the supply current will reduce

to 0.1

µA or less.

The continuous output current of the SS6550

can be up to 800mA and the output voltage can be

adjusted from 0.75V to VIN with an input range

from 2.5V to 5.5V using a voltage divider. The

SS6550 also features high efficiency, low drop-

out voltage, and a 0.75V reference with

±1.2% ac-

curacy. It is available in a space-saving 8-pin

MSOP package.

Operation

When powered on, the control logic block detects

whether the SYNC/MODE pin is connected to VIN or

GND to determine the operation function and gives a

signal to the PWM/PFM control block to determine the

proper

comparator

(ref.

Block

Diagram).

The

SS6550 works with an internal synchronous rectifier

Q3, to increase efficiency. When the control logic block

turns Q2 on, Q3 will turn off through the anti-short-

through block. Similarly, when Q3 is on, Q2 will turn off.

The SS6550 provides a current limit function by us-

ing a 5

Ω resistor. When Q1 turns on, current flows

through the 5

Ω resistor and the current amplifier

senses the voltage across the resistor and amplifies it.

When the sensed voltage gets bigger than the refer-

ence voltage, the control logic shuts the device off.

PWM/PFM Function

When connecting the SYNC/MODE pin to VIN, the de-

vice is forced into the PWM (Pulse-Width-Modulated)

mode with constant frequency. The advantage of con-

stant frequency is that noise can be reduced easily

without complex post-filtering. However, it has the dis-

advantage of low efficiency at light loading. Therefore,

the SS6550 provides a function to solve this prob-

lem. When connecting the SYNC/MODE pin to GND,

the device is able to get into PWM/PFM (Pulse-

Frequency-Modulated) modes. Under a light load con-

dition, the device shifts to PFM mode, which results in

a higher efficiency. PWM mode is on under heavy

loading and the noise is reduced.

Frequency Synchronization

Connecting

an

external

clock

signal

to

the

SNYC/MODE pin can control the switching frequency.

The acceptable range is from 500 kHz to 1 MHz. This

mode exhibits low output ripple as well as low audio

noise and reduces RF interference, while providing

reasonable low current efficiency.

Adjustable Switching Frequency

The decrease of the switching frequency can also be

controlled by connecting an external resistor from the

RT pin to ground (ref. Fig. 15). In this mode, the PFM

mode is disabled and the device operates with the ad-

justed frequency. This function is helpful in reducing

high frequency harmonics and a post-filter can be eas-

ily designed for this function. However, there will be an

increase in ripple voltage.

Rev.2.01 6/06/2003