LT1302 Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LT1302

Micropower High Output Current Step-Up Adjustable and Fixed 5V DC/DC Converters

Linear Technology 

LT1302/LT1302-5

BLOCK DIAGRA SM

SENSE

4

R1

315k

1.24V

REFERENCE

SHDN 3

R2

105k

SHUTDOWN

VIN

6

36mV

+

–

VOS

15mV

CMP1

ENABLE

OFF

220kHz

OSCILLATOR

HYSTERETIC

COMPARATOR 2µA

VIN

Q5

–

A1

+

ERROR

AMPLIFIER

A2

R5

730Ω

A3

DRIVER

VIN

Q1 Q2

BIAS

300Ω 3.6k

SW

7

R4

1.75Ω

Q3 Q4

160X

1

GND

2

5

8

VC

IT

1302 F03

PGND

Figure 3. LT1302-5 Block Diagram

U

OPERATIO

The LT1302’s operation can best be understood by

examining the block diagram in Figure 2. The LT1302

operates in one of two modes, depending on load. With

light loads, comparator CMP1 controls the output; with

heavy loads, control is passed to error amplifier A1.

Burst Mode operation consists of monitoring the FB pin

voltage with hysteretic comparator CMP1. When the FB

voltage, related to the output voltage by external attenu-

ator R1 and R2, falls below the 1.24V reference voltage,

the oscillator is enabled. Switch Q4 alternately turns on,

causing current buildup in inductor L1, then turns off,

allowing the built-up current to flow into output capaci-

tor C3 via D1. As the output voltage increases, so does

the FB voltage; when it exceeds the reference plus

CMP1’s hysteresis (about 5mV) CMP1 turns the oscilla-

tor off. In this mode, peak switch current is limited to

approximately 1A by A2, Q2, and Q3. Q2’s current, set at

34µA, flows through R5, causing A2’s negative input to

be 25mV lower than VIN. This node must fall more than

36mV below VIN for A2 to trip and turn off the oscillator.

The remaining 11mV is generated by Q3’s current flow-

ing through R4. Emitter-area scaling sets Q3’s collector

current to 0.625% of switch Q4’s current. When Q4’s

current is 1A, Q3’s current is 6.25mA, creating an 11mV

drop across R4 which, added to R5’s 25mV drop, is

enough to trip A2.

When the output load is increased to the point where the

1A peak current cannot support the output voltage,

6

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