lt1949-1 Linear Technology Corporation, lt1949-1 Datasheet - Page 6

• Manufacturer Part Number: lt1949-1
• Description: 1.1mhz, 1a Switch Pwm Dc/dc Converter
• Manufacturer: Linear Technology Corporation

OPERATIO
APPLICATIO S I FOR ATIO
LT1949-1
Low-Battery Detector
The LT1949-1’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
ment as a low-battery detector is straightforward.
6
5% reference. The positive input is the LBI pin. Arrange-
Figure 5. Setting Low-Battery Detector Trip Point
R1
R2
49.9k
LBI
GND
U
200mV
INTERNAL
REFERENCE
U
+
–
V
IN
U
LT1949-1
1949-1 F05
LBO
Figure 4. Recommended Component Placement for Boost
Converter. Note Direct High Current Paths Using Wide PC
Traces. Minimize Trace Area at Pin 1 (V
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
GROUND PLANE
W
SHUTDOWN
3.3V
MULTIPLE
R1
VIAs
1M
R1 =
GND
R2
TO PROCESSOR
V
LB
U
– 200mV
4 A
1
2
3
4
LT1949-1
C2
LBI
LBO
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, 49.9k is adequate. The
200mV reference can also be accessed as shown in
Figure 6. The low-battery detector remains active in
shutdown.
8
7
6
5
V
C1
OUT
C
) and Pin 2 (FB).
Figure 6. Accessing 200mV Reference
200mV
V
2N3906
REF
V
L1
IN
1949-1 F04
10k
+
200k
10 F
LBO
LBI
LT1949-1
GND
V
IN
1949-1 F06