Linear Design Features - LTMag-V18N02-07-LT3507 - Michael Nootbaar User Manual

DESIGN FEATURES L

Quad Output Regulator Meets Varied Demands of Multiple Power Supplies

by Michael Nootbaar Introduction

Many modern electronic devices re- generated as efficiently as possible to to 2.4A of output current. Channels 2 quire a number of power domains to minimize power consumption. and 3 are each capable of providing up satisfy the needs of a wide variety of de- The LT3507 meets these require- to 1.6A of output current. Each of the vices and subsystems. A power supply ments by combining three switching three switching regulators has its own designer’s job would be relatively easy regulators and a low dropout linear input supply pin to the power switch. if the design contraints were limited to regulator in a compact 5mm × 7mm The regulators may be operated from simply providing well-regulated volt- QFN package. The switching regula- different supplies in order to maximize ages, but power supply requirements tors have internal power switches, system efficiency. are typically much more complicated. independent input supplies, run and The maximum voltage on any of the For example, multiple power rails must track/soft-start controls, and power VIN pins is 36V. The LT3507 internal be sequenced and/or track each other good indicators. The LDO requires circuitry is powered from VIN1, which to ensure proper system behavior. an external NPN pass transistor and requires a minimum operating voltage High power sections of the design includes track/soft-start control. for VIN1 of 4V. The minimum operating are often powered down when not voltage for VIN2 and VIN3 is 3V. Since

in use, requiring multiple shutdown Three Independent VIN1 powers the internal circuitry, it options. Powering analog circuitry Switching Regulators… must always be at least 4V when any

adds the demand for clean, low noise The LT3507 includes three indepen- channel is running, even if Channel 1 supplies—no switching transients dent, monolithic switching regulators is off. or excessive voltage ripple allowed. to achieve a space saving solution. All three regulators use a cur- And, of course, all supplies must be Channel 1 is capable of providing up rent mode, constant frequency

VIN 6V TO 36V

CMDSH-4E VIN1 VIN2 VIN3 VINSW

VOUT2

BOOST1 UVLO

4.7µHVOUT1 0.22µF

VOUT1

1.8V SW1 OVLO

FB1 680p F 100k 100k 100k

VC1 PGOOD1 PGOOD1 15k PGOOD2 PGOOD2

18.7k PGOOD3 PGOOD3

15k LT3507 CMDSH-4E TRK/SS1 BOOST2

CMDSH-4E 10µH

0.22µF VOUT2

18.7k VOUT2 BOOST3 SW2 3.3V

0.22µF FB2

VOUT3 15µH

5V SW3 1000p F

FB3 VC2 22µF

TRK/SS2

TRK/SS2 TRK/SS2

TRK/SS3

DRIVE Q1

VOUT4

RUN2 FB4

SHDN RUN3

SYNC SOURCE 1k 0.2A

POWERED FROM RT/SYNC TRK/SS4

3.3V OUTPUT, VOUT2 GND

BAS70 105k

f SW = 450k Hz

L1: WÜRTH WE-PD 744 778 9004 L2: WÜRTH WE-PD 744 778 9115 L3: WÜRTH WE-PD 744 778 910 D1, D2, D3: DIODES, INC. B240A Q1: ON SEMICONDUCTOR NSS30101LT1G

Figure 1. The LT3507 in a wide input range, quad output application

Linear Technology Magazine • June 2008