5

LT1614

Shutdown Pin

The LT1614 has a Shutdown pin (SHDN) that must be

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

shown in Figure 1.

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

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

tion, a benign condition.

Figure 1. Shutdown Circuit

Q3

SHUTDOWN

CURRENT

R2

400k

200k

Q2

1614 F01

Q1

START-UP

CURRENT

SHDN

Low-Battery Detector

The LT1614’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

reference. The positive input is the LBI pin. Arrangement

as a low-battery detector is straightforward. Figure 2

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 3.

Coupled Inductors

The applications shown in this data sheet use two un-

coupled inductors because the Murata units specified are

small and inexpensive. This topology can also be used

with a coupled inductor as shown in Figure 4. Be sure to

get the phasing right.

LBO

LBI

TO PROCESSOR

R1

1M

R2

100k

LT1614

1614 F02

3.3V

GND

200mV

INTERNAL

REFERENCE

–

+

R1 =

2

µA

Figure 2. Setting Low-Battery Detector Trip Point

LT1614

LBI

LBO

200k

10

µF

GND

10k

1614 F03

2N3906

200mV

+

Figure 3. Accessing 200mV Reference

SW

LT1614

L1A

10

µH

5V

D1

NFB

SHDN

69.8k

24.9k

100k

1nF

– 5V

200mA

C1, C2: AVX TAJB336M010

C3: AVX 1206CY106

D1: MBR0520

L1: COILTRONICS CTX10-1

1614 F04

C1

33

µF

C2

33

µF

GND

L1B

10

µH

C3

1

µF

••

+

Figure 4. 5V to – 5V Converter with Coupled Inductor