LT1307 datasheet(13/20 Pages) LINER

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LT1307/LT1307B

APPLICATIONS INFORMATION

tions where small size is not critical, Panasonic HFQ series

aluminum electrolytic capacitors have been found to per-

form well.

Table 2. Vendor Telephone Numbers

VENDOR

COMPONENTS

TELEPHONE

Coilcraft

Inductors

(708) 639-6400

Marcon

Capacitors

(708) 913-9980

Murata-Erie

Inductors, Capacitors

(404) 436-1300

Sumida

Inductors

(847) 956-0666

Tokin

Capacitors

(408) 432-8020

AVX

Capacitors

(207) 282-5111

Sprague

Capacitors

(603) 224-1961

Coiltronics

Inductors

(407) 241-7876

Diodes

Most of the application circuits on this data sheet specify

the Motorola MBR0520L surface mount Schottky diode.

This 0.5A, low drop diode complements the LT1307 quite

well. In lower current applications, a 1N4148 can be used,

although efficiency will suffer due to the higher forward

drop. This effect is particularly noticeable at low output

voltages. For higher voltage output applications, such as

LCD bias generators, the extra drop is a small percentage

of the output voltage so the efficiency penalty is small. The

low cost of the 1N4148 makes it attractive wherever it can

be used. In through hole applications the 1N5818 is the all

around best choice.

SHUTDOWN PIN

The LT1307 has a Shutdown pin (SHDN) that must be

grounded to shut the device down or tied to a voltage equal

or greater than VIN to operate. The shutdown circuit is

shown in Figure 13.

Note that allowing SHDN to float turns on both the start-

up current (Q2) and the shutdown current (Q3) for VIN >

2VBE. The LT1307 doesn’t know what to do in this situation

and behaves erratically. SHDN voltage above VIN is al-

lowed. This merely reverse-biases Q3’s base emitter junc-

tion, a benign condition.

Figure 13. Shutdown Circuit

VIN

SHUTDOWN

CURRENT

400k

200k

1307 F13

START-UP

CURRENT

SHDN

LOW-BATTERY DETECTOR

The LT1307’s low-battery detector is a simple PNP input

gain stage with an open collector NPN output. The nega-

tive input of the gain stage is tied internally to a 200mV

±5% reference. The positive input is the LBI pin. Arrange-

ment as a low-battery detector is straightforward. Figure

14 details hookup. R1 and R2 need only be low enough in

value so that the bias current of the LBI pin doesn’t cause

large errors. For R2, 100k is adequate. The 200mV refer-

ence can also be accessed as shown in Figure 15.

LBO

LBI

TO PROCESSOR

100k

VIN

LT1307

1307 F14

3.3V

GND

200mV

INTERNAL

REFERENCE

–

R1 =

VLB – 200mV

µA

Figure 14. Setting Low-Battery Detector Trip Point

VIN

LT1307

LBI

LBO

200k

µF

GND

10k

1307 F15

2N3906

VREF

200mV

Figure 15. Accessing 200mV Reference

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