6

LT1949

Low-Battery Detector

The LT1949’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 5 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 reference can also be accessed as shown in

Figure 6. The low-battery detector remains active in

shutdown.

1

2

8

7

3

4

6

5

L1

C2

LBO

LBI

LT1949

GND

SHUTDOWN

R1

R2

MULTIPLE

VIAs

GROUND PLANE

1949 F04

C1

+

Figure 4. Recommended Component Placement for Boost

Converter. Note Direct High Current Paths Using Wide PC

Use Multiple Vias to Tie Pin 4 Copper to Ground Plane. Use

Vias at One Location Only to Avoid Introducing Switching

Currents into the Ground Plane

LBO

LBI

TO PROCESSOR

R1

1M

R2

100k

LT1949

1949 F05

3.3V

GND

200mV

INTERNAL

REFERENCE

–

+

R1 =

2

µA

Figure 5. Setting Low-Battery Detector Trip Point

LT1949

LBI

LBO

200k

10

µF

GND

10k

1949 F06

2N3906

200mV

+

Figure 6. Accessing 200mV Reference

OPERATIO