LINEAR Application Note 58 5V to 3.3V Converters for Microprocessor Systems

Application Note 58 September 1993

5V to 3.3V Converters for Microprocessor Systems Robert Dobkin, Mitchell Lee, Dennis O'Neill and Milt Wilcox

Introduction Linear Regulators

The new generation of high performance microproces- Table 1 shows the range of components available for linear sors are built on dense, low breakdown voltage processes regulation of 3.3V with a 5V input. With only 1.7V of in order to accommodate increased transistor counts. headroom, low dropout is essential. Low dropout regula- These new processors require high current power at 3.3V, tors are available delivering currents from 125m A to 7.5A, developed from the 5V input used to power the rest of the allowing almost any microprocessor to be powered with a system. Special techniques are required to ensure proper local 3.3V generation circuit. The first four devices (LT1020, operation of the microprocessor and good heat dissipa- LT1120, LT1121 and LT1129) are PNP micropower low tion within the computer system. dropout regulators. Since PNP transistors are much larger

than monolithic NPNs, higher current regulators use an

The 3.3V supply may be either a linear or switching type.

NPN pass device. The LT1117, LT1086, LT1083 through

For most applications a linear regulator is preferable since

LT1085, and LT1087 all use NPN pass devices. The NPN

it minimizes components and has acceptable efficiency for

structure requires about 1.2V headroom compared to the

a whole computer system. In portable computers where

400m V to 500m V dropout typical of PNP regulators, and

high efficiency is paramount because of battery operation,

ground current of 5m A or 10m A, independent of output

a switching supply is necessary.

current. Because of this constant quiescent current, the

This application note contains a collection of 3.3V regula- LT1117 and LT1083 family are not suitable for applica- tor circuits, each optimized for a 5V input and surface tions requiring micropower standby. mount technology. The circuits are split into two catego-

Table 1. 3.3V Linear Regulators

ries, linear and switching, and further arranged by current

capability. LOAD PASS SHUTDOWN CURRENT DEVICE DEVICE CURRENT TOLERANCE*

Most of the circuits, with the possible exception of the high

125m A LT1020 PNP 40µA 2.1% (69m V)

current linear regulators, are surface mountable. Where

125m A LT1120 PNP 40µA 2.1% (69m V)

appropriate, part numbers are given for surface mount

150m A LT1121-3.3 PNP 16µA 3% (100m V)

coils, capacitors, and diodes. Resistors and small capaci-

700m A LT1129-3.3 PNP 16µA 3% (100m V)

tors, unless there are special characteristics, are generic

800m A LT1117-3.3 NPN 2% (65m V)

and manufacturer’s part numbers are not shown.

1.5A LT1086-3.3 NPN 1.6% (53m V)

Both linear and switching regulators are available for the 3A LT1085-3.3 NPN 1.6% (53m V) purpose of converting 5V to 3.3V. In general, the linear 5A LT1084 NPN 1.9% (61m V) regulators are the best choice at lower (≤3A) current

7.5A LT1083 NPN 1.9% (61m V)

levels where their dissipation is minimized, or in line-

10A 2 × LT1087 NPN 1.9% (61m V)

operated equipment where 66% theoretical efficiency is

*Includes line, load, and temperature variations. Adjustable parts also

acceptable. Switchers are favored in higher current and include worst case effect of external 1% resistors. efficiency-conscious applications. Efficiencies in the 90% to 95% range are the norm for switchers described in this application note.

AN58-1