7

LT1573

Figure 2. Basic Regulator Circuit

on the regulator circuit under overload conditions. The

of the circuit, primarily the dropout voltage and the output

current. The dropout voltage of an LT1573-based regula-

the discrete external PNP transistor when it is driven with

a base current equal to the available drive current of the

LT1573.

External PNP Transistor Selection Criteria

The selection of an appropriate external PNP transistor

depends on the regulator application specifications. The

critical PNP transistor selection criteria include:

1. The maximum output current of the PNP transistor

2. The dropout voltage at the maximum output current

The PNP transistor must be able to supply the specified

maximum regulator output current to be qualified for the

transistor at the maximum output current determines the

dropout voltage of the circuit. The dropout voltage deter-

mines the minimum regulator input voltage for a certain

of the transistor determines how fast the voltage regulator

can follow an output load change without losing voltage

regulation.

The D45H11 from Motorola and the KSE45H11TU from

Samsung can be used in all LT1573 regulator circuits with

current ratings up to 5A. The D45H11 can supply 5A of

COMP

DRIVE

FB

LATCH

SHDN

GND

LT1573

+

GND

1573 F02

R1

R2

LOAD

+

+

The LT1573 is designed to be used in conjunction with an

external PNP transistor. The overall specifications of a

regulator circuit using the LT1573 and an external PNP will

be heavily dependent on the specifications of the external

PNP. While there are a wide variety of PNP transistors

available that can be used with the LT1573, the specifica-

tions given in a typical transistor data sheet are of little use

in determining overall circuit performance. In the follow-

ing discussion the critical requirements of the PNP tran-

sistors are noted. Design equations are given and

examples are shown using a readily available discrete PNP

transistor. This device is inexpensive, available from mul-

tiple sources and can be used for a wide range of applica-

tions. For applications using other PNP transistors, the

regulator specifications can be derived by the same method.

Basic Regulator Circuit

The basic regulator circuit is shown in Figure 2. The

adjustable output LT1573 senses the regulator output

voltage from its feedback pin via the output voltage

divider, R1 and R2, and drives the base of the external PNP

transistor to maintain the regulator output at the desired

value. For fixed output versions of the LT1573, the regu-

lator output voltage is sensed from the feedback pin via an

drive current available to the external PNP transistor and

to limit the power dissipation in the LT1573. Limiting the

drive current to the external PNP transistor will limit the

output current of the regulator which minimizes the stress

APPLICATIO S I FOR ATIO