LT1573C 查看數據表（PDF） - Linear Technology

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LT1573C

Low Dropout PNP Regulator Driver

Linear Technology 

LT1573

APPLICATIO S I FOR ATIO

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

internal voltage divider. The resistor RD is required for the

overcurrent latch-off function. RD is also used to limit the

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

on the regulator circuit under overload conditions. The

resistor RD is chosen based on the operating requirements

of the circuit, primarily the dropout voltage and the output

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

tor circuit is determined by the VCE saturation voltage of

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

3. The gain-bandwidth product fT of the transistor

The PNP transistor must be able to supply the specified

maximum regulator output current to be qualified for the

regulator application. The VCE saturation voltage of 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

specified output voltage. The gain-bandwidth product fT

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

+

CTIME

VIN

FB

COMP

LATCH

VOUT

LT1573

SHDN

VIN

GND

DRIVE

CC

RC

RD

RB

QOUT

R1

++

CIN

COUT1

COUT2 R2

Figure 2. Basic Regulator Circuit

VOUT

LOAD

GND

1573 F02

7

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