Datasheet
6
LT1949
APPLICATIONS INFORMATION
WUU
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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
V
OUT
V
IN
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
Traces. Minimize Trace Area at Pin 1 (V
C
) and Pin 2 (FB).
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
V
IN
LT1949
1949 F05
3.3V
GND
200mV
INTERNAL
REFERENCE
–
+
R1 =
V
LB
– 200mV
2µA
Figure 5. Setting Low-Battery Detector Trip Point
V
IN
LT1949
LBI
LBO
200k
10µF
GND
10k
1949 F06
2N3906
V
REF
200mV
+
Figure 6. Accessing 200mV Reference
OPERATIO
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