STw4141

3 Functional Description

17/27

3

Functional Description

3.1

Introduction

The STw4141 is an easy to use, single coil dual outputs step down DC/DC converter optimized

to supply low-voltage to CPUs or DSPs in cell phones and other miniature devices powered by

single cell lithium-ion or 3 cell NiMH/NiCd batteries. It provides two different output voltages

with high efficiency operation in a wide range of output currents. The device offers high DC

voltage regulation accuracy and load transient response to satisfy demanding processor core

supply. The converter is based on voltage mode buck architecture using PWM and PFM

operation modes.

At light load currents, the device can operate in PFM mode to maintain high efficiency over the

entire load current range. Switching between PWM and PFM modes can be done automatically

or can be forced by external pins (AUTO and MODE/SYNC). Externally synchronized or fixed

frequency (internal oscillator) PWM mode offers full output current capability while minimizing

interference to sensitive RF and data acquisition circuits.

3.2

PWM and PFM mode operation

PWM (Pulse Width Modulation) mode is intended for normal load to high load currents. Energy

is delivered to the load with an accurate and defined frequency of 900 kHz.

PFM (Pulse Frequency Modulation) mode is intended for low load currents to maintain high

efficiency conversion.

Forced mode: When AUTO pin is LOW, the operating mode is selectable by the user itself. It

means that system controls the behavior of the STw4141 according to processor needs.

STw4141 is switched from PWM to PFM mode using MODE/SYNC pin (refer to

User mode

details” section).

Automatic PWM / PFM switch: When AUTO pin is HIGH, the operating mode is directly

controlled by internal digital circuit according to processor needs. The device switches from

PWM to PFM by itself if sum of output currents is lower than approximately 100 mA during at

least 16 clock cycles. The device can be forced to PWM mode connecting MODE/SYNC to

HIGH level. (see

Section 2.8 and Section 2.9).

3.3

Current limiter

This protection limits the current flowing through coil. As soon as ILIM is detected, the duty

cycle is terminated and prevents the coil current against rising above peak current limit. There

is no reset of device.

3.4

Short circuit protection

It protects the device against short-circuit at output terminals. When one or both output

voltages are decreased by 0.7 V below their nominal output values the device enters into reset

followed by soft start sequence.