LINEAR LTC3100 User Guide

L DESIGN IDEAS

Multi-Rail DC/DC Converter in a 3mm × 3mm QFN Takes

Inputs as Low as 0.7V by Dave Salerno

Introduction

MBR0520 Modern handheld instrumentation, USB 3.3V AT: 100m A FOR VBATT = 1.2V INPUT 300m A FOR VBATT = 2.4V

portable medical devices and con-

4.7µF 250m A FOR USB INPUT 3.3µH

sumer electronics demand a multitude

VBATT

of power rails for internal processors, 4.7µF

0.9V TO 1.07M

SWBST VINBK VBST

memory, audio and color displays.

VINBST FBBST

EF FI CI EN CY

Popular battery technologies for these 4.7µF 324k

devices include single or multiple VLDO

cell alkaline, Ni MH or Li-Poly/Li-Ion 301k

LTC3100

batteries. Operation from a USB port FBLDO

1.8V AT 50m A

or a wall adapter is another common 100k

trait. Replaceable alkaline batteries 2.2µF

are particularly attractive for remote 10µH

MODE SWBK

locations and portable medical devices

where power or time is not available RUNBST FBBK

to recharge batteries. The challenge RUNLDO PGBST is to create a compact and efficient

RUNBK GND PGBK

power solution for these wide VIN

range, multi-output applications. The LTC3100 multichannel DC/DC converter makes it easier to meet this

Figure 1. Multiple-input source 3.3V and 1.8V converter

challenge. The LTC3100 is a high efficiency, and 5.25V. Both converters feature The circuit shown in Figure 1 takes 1.5MHz multichannel DC/DC con- automatic Burst Mode® operation for advantage of the LTC3100’s ability verter in a compact 3mm × 3mm × high efficiency at light loads. For low to operate from independent input 0.75mm QFN package. It features a noise applications, Burst Mode opera- sources to generate multiple outputs synchronous step-up (boost) DC/DC tion can be disabled by grounding the regardless of which power source is converter, a synchronous step-down MODE pin. The 100m A LDO, whose available. In this example, the boost (buck) DC/DC converter and a 100m A input is internally connected to the converter produces a regulated 3.3V low dropout linear regulator (LDO). boost output, can be used to produce output from a single- or dual-cell input The boost and buck converters can a third, low noise output. It can also voltage. The buck converter runs from operate independently from differ- be used for voltage sequencing of the a 5V USB or wall adapter input and ent sources, from the same source, boost output voltage. produces 3.3V as well, with its output or cascaded to create a buck-boost externally tied to that of the boost. converter. Internal loop compensation By leaving the MODE pin open,

simplifies the design and minimizes 3.3V OUTPUT the automatic Burst Mode operation

external component count and so- feature of the LTC3100 is enabled,

VIN = 2.4V

lution size. Each converter has a maximizing battery life at light load.

VIN = 1.2V

Power Good indicator that is useful When USB or wall adapter power is for voltage sequencing. The boost available, the buck converter is au- converter offers up to 95% efficiency tomatically enabled and generates a

VIN = 5V USB

and features true output disconnect, 3.3V output that is set 3.8% higher a 700m A minimum current limit and than that of the boost converter. This can start with input voltages as low puts the boost converter in sleep mode, as 700m V, making it ideal for single reducing the load on the batteries to

alkaline or Ni MH cell applications. 0.01 0.1 just a few micro-amps. The result is

LOAD CURRENT (m A)

The buck converter offers up to 94% a 3.3V output that seamlessly tran- efficiency and can deliver 250m A or sitions from battery power to USB

Figure 2. Converter efficiency

more from input voltages between 1.8V for the circuit of Figure 1 continued on page 37

Linear Technology Magazine • June 2008