LINEAR dn435f Data Sheet & Manual

Single Resistor Sets Positive or Negative Output for DC/DC Converter – Design Note 435 Jesus Rosales Introduction the use of tiny component sizes, reducing the real estate

Many electronic subsystems, such as VFD (vacuum required and the output ripple. If power loss is a concern, fl ourecent display), TFT-LCD, GPS or DSL applications, switching at a lower frequency reduces switching losses, require more than just a simple step-down or step-up improving effi ciency. The switching frequency can be set DC/DC converter. They may require inverting, noninverting from 200k Hz to 2.5MHz via a single resistor from the RT converters or both. Designers usually resort to different pin to ground. The device can also be synchronized to an regulator ICs to control various polarity outputs, thus external clock via the SYNC pin.

increasing the inventory list. The LT®3580 solves this

Soft-Start and Undervoltage Lockout

problem by controlling either positive or negative outputs

To alleviate high inrush current levels during start-up, the

using the same feedback confi guration. It contains an in-

LT3580 includes a soft-start feature which controls the

tegrated 2A, 42V switch and packs many popular features

ramp rate of the switch current by the use of a capacitor

such as soft-start, adjustable frequency, synchronization

from SS to ground.

and a wide input range into a small footprint. The LT3580 comes in an 8-pin 3mm × 3mm DFN or MSOP packages The SHDN pin in the LT3580 serves two purposes. Tying and can be used in multiple confi gurations such as boost, it high or low turns the converter on or off. In situations SEPIC, fl yback and Cuk topologies. where the input supply is current limited, has a high source

impedance or ramps up/down slowly, the SHDN pin can

Sensing Output Voltage Has Never Been Easier

be confi gured to provide undervoltage lockout through

The LT3580 has a novel FB pin architecture that simpli-

a simple resistor divider from VIN to ground.

fi es the design of inverting and noninverting topologies.

Namely, there are two internal error amplifi ers; one senses Boost Converters

positive outputs and the other negative. Additionally, the A boost converter, shown in Figure 1, produces a posi- LT3580 has integrated the ground side feedback resistor tive output voltage always higher than its input. Figure 2 to minimize component count. To illustrate the benefi ts, shows the effi ciency graph for the boost converter in notice how the schematics in Figures 1, 3 and 5 need Figure 1 at a 4.2V input. only one feedback resistor.

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A single sense resistor simply connects to the FB pin

on one side and to the output on the other regardless of L1

the output polarity, eliminating the confusion associated DFLS220L VOUT

VIN CDRH5D16-6R8

with positive or negative output sensing and simplifying 3V TO

C2 300m A AT 3VIN

the board layout. A user decides the output polarity he 4.7μF R2 500m A AT 4.2VIN

needs, the topology he wants to use and the LT3580 VIN SW

FBSHDN C4

does the rest.

LT3580 10μF

SYNC VC

Adjustable/Synchronizable Switching Frequency

SS RT GND

R5 It is often necessary to operate a converter at a particular 20k

frequency, especially if the converter is used in an RF com-

munications product that is sensitive to spectral noise in

DN435 F01

certain frequency bands. Also, if the area available for a converter is limited, operating at higher frequencies allows Figure 1. 3V-10V to 12V, 300m A Boost Converter