Reference DesignAnalog

Single LTC1149 Provides 3.3V and 5V in Surface Mount (Design Note 77)

Design Note 77 describes a 17W dual-output power supply using a single LTC1149 to regulate 3.3V and 5V rails with efficiencies exceeding 90% across a 6V to 24V input range.

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Overview

This design document details a compact surface mount power solution utilizing the LTC1149 synchronous step-down controller to provide simultaneous 3.3V and 5V regulation. The circuit supports a 17W capability and operates from an input voltage range of 6V to 24V. It features an AC-coupled gate drive for low-voltage enhancement and a diode-capacitor network to ensure synchronized power-up for both output rails. The document provides performance data on cross-regulation and efficiency, along with layout considerations for noise sensitivity and component selection for various power levels. Detailed schematic diagrams and part recommendations for surface mount transformers and electrolytic capacitors are included.

Use Cases

Battery-powered systems requiring dual 3.3V and 5V rails
Disk drive startup and surge load management
Compact industrial power supplies
Portable computing power management

Topics

LTC1149

LTC1148

Si9435DY

Si9410DY

DC-DC converter

synchronous regulator

dual output

3.3V regulator

5V regulator

surface mount design

BAS16

MBRS140

Referenced Parts

10SA220M

Sanyo

On the outputs, Sanyo 10SA220M OS-CON capacitors (220μF, 10V) provide excellent performance

HM00-93839

Beckman Industrial Corporation

Beckman Industrial Corporation has developed a very low profile, surface mount transformer suitable for applications where V IN ≥ 9V (HM00-93839).

MBRS140

Onsemi

D4 MBRS140

HL-8700

Hurricane Electronics Lab

a Hurricane Electronics Lab throughhole transformer (HL-8700) was used to reduce overall height.

LTC1149

Linear Technology

This Design Note describes a circuit which uses one LTC 1149 to regulate both a 3.3V and a 5V output

77050-A7

Magnetics, Inc.

11 T TRIFILAR AWG #23 WIRE ON MAGNETICS, INC. 77050-A7 CORE.

Si9410DY

Siliconix

QN1 Si9410DY

Si9435DY

Siliconix

QP1 Si9435DY

LTC1148

Linear Technology

In low voltage applications, the LTC1148 can be substituted for the LTC1149

BAS16

NXP Semiconductors

D3 BAS16

10SA220M	Sanyo	On the outputs, Sanyo 10SA220M OS-CON capacitors (220μF, 10V) provide excellent performance
HM00-93839	Beckman Industrial Corporation	Beckman Industrial Corporation has developed a very low profile, surface mount transformer suitable for applications where V IN ≥ 9V (HM00-93839).
MBRS140	Onsemi	D4 MBRS140
HL-8700	Hurricane Electronics Lab	a Hurricane Electronics Lab throughhole transformer (HL-8700) was used to reduce overall height.
LTC1149	Linear Technology	This Design Note describes a circuit which uses one LTC 1149 to regulate both a 3.3V and a 5V output
77050-A7	Magnetics, Inc.	11 T TRIFILAR AWG #23 WIRE ON MAGNETICS, INC. 77050-A7 CORE.
Si9410DY	Siliconix	QN1 Si9410DY
Si9435DY	Siliconix	QP1 Si9435DY
LTC1148	Linear Technology	In low voltage applications, the LTC1148 can be substituted for the LTC1149
BAS16	NXP Semiconductors	D3 BAS16