LINEAR Compact Triple Step-Down Regulator with LDO Driver, Output Tracking, and Sequencing – Design Note 441 by Tiger Zhou Owner's Manual & User Guide

Compact Triple Step-Down Regulator Offers LDO Driver and Output Tracking and Sequencing – Design Note 441 Tiger Zhou

Introduction VIN 6V TO 36V 22μF

Typical industrial and automotive applications require

VIN1 VIN2 VIN3 100k

multiple high current, low voltage power supplies to PGOOD1 PGOOD2

CMDSH-3 CMDSH-3

drive everything from disk drives to microprocessors. PGOOD3 BOOST2BOOST1

The LT®3507 triple step-down converter fi ts easily into

VOUT1 4.7μH 0.22μF 0.22μF 8.2μH VOUT2

1.8V SW1 SW2 3.3V

these applications. It is simple and compact compared B340A B240A

to multi-chip solutions.

FB1 FB2

47μF 10k 680p F LT3507 680p F 10k

The LT3507 is a single IC current mode triple step- 20k 20k

VC1 VC2

down regulator with internal power switches and a low CMDSH-3

dropout linear regulator driver. The switching convert- BOOST3 BIAS

VOUT3 10μH 0.22μF

ers are capable of generating one 2.4A output and two 5V SW3 DRIVE

1.5A outputs. All three converters are synchronized to a

53.6k 21.5k VOUT4 single oscillator, with the 2.4A output running antiphase FB3 FB4 2.5V 680p F 0.2A

to the other two converters, thereby reducing input ripple 20k

current. Each converter has independent shutdown and

RUN1 TRK/SS1 SHDN RUN2 TRK/SS2

soft-start circuits and generates a power good signal

RUN3 TRK/SS3 RT/SYNC TRK/SS4

when its output is in regulation, simplifying both supply

GND 0.22μF

sequencing and the interface with microcontrollers and DN441 F01

DSPs. Separate input pins for each regulator offer ad-

Figure 1. A 4-Output Supply Including a Low Noise LDO

ditional fl exibility; regulators can be cascaded to reduce circuit size, or each regulator can draw power from a

noise rail. Figure 1 shows a typical application for four

different input source.

outputs—1.8V at 2.4A, 3.3V at 1.3A, 5.0V at 1.5A and The switching frequency is set with a single resistor 2.5V at 0.2A—from a 6V-36V input supply.

between 250k Hz to 2.5MHz. High switching frequency

Low Ripple High Frequency Operation Even at High

allows the use of small inductors and capacitors result-

VIN/VOUT Ratios

ing in a very compact triple output supply. The constant

High frequency operation minimizes solution size, but

switching frequency, combined with low impedance

one obstacle to high voltage (36V), high frequency

ceramic capacitors, results in low output ripple. With

(MHz) operation of monolithic step-down regulators is

its wide input voltage range of 4V to 36V, the LT3507

the minimum on-time constraint. Due to an internal logic

regulates a broad array of power sources including 5V

propagation delay, a step-down regulator must remain

logic rails, unregulated wall transformers, lead acid bat-

on for a minimum time interval for proper operation.

teries and distributed power supplies.

Otherwise, the converter operates in pulse-skipping mode

6V to 36V Input to Four Outputs—1.8V, 3.3V, 5V at high input-to-output ratios, which has the undesirable

and 2.5V—One IC side effect of increasing output ripple. For example, the The triple converter accommodates a 6V to 36V input application in Figure 1 best operates at 450k Hz when the voltage range and is capable of supplying up to 2.4A, input is 36V and the output is 1.8V.

1.5A and 1.5A, respectively. The 20m A LDO driver out-

L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

put can drive an NPN transistor to provide a fourth low All other trademarks are the property of their respective owners.