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LT3491
APPLICATIO S I FOR ATIO
LOW INPUT VOLTAGE APPLICATIONS
The LT3491 can be used in low input voltage applications.
The input supply voltage to the LT3491 must be 2.5V or
higher. However, the inductor can be run off a lower
battery voltage. This technique allows the LEDs to be
powered off two alkaline cells. Most portable devices have
a 3.3V logic supply voltage which can be used to power the
LT3491. The LEDs can be driven straight from the battery,
resulting in higher efficiency.
Figure 10 shows three LEDs powered by two AA cells. The
battery is connected to the inductor and the chip is
BOARD LAYOUT CONSIDERATIONS
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To prevent electromagnetic interference (EMI) problems,
proper layout of high frequency switching paths is essen-
tial. Minimize the length and area of all traces connected to
the switching node pin (SW). Keep the sense voltage pins
(CAP and LED) away from the switching node. Place C OUT
next to the CAP pin. Always use a ground plane under the
switching regulator to minimize interplane coupling. Rec-
ommended component placement is shown in Figure 11.
powered off a 3.3V logic supply voltage.
SHUTDOWN AND
DIMMING CONTROL
3.3V
C1
0.1 μ F
V IN
CTRL
CAP
2 AA CELLS
2V TO 3.2V
L1
10 μ H
SW
LT3491
GND
LED
R SENSE
10 ?
C2
2.2 μ F
C1
1 μ F
3491 F10
C1: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN GMK325BJ225ML
L1: MURATA LQH32CN100
Figure 10. 2 AA Cells to Three White LEDs
C IN
CTRL
V IN
GND
5
4
C IN
LED
R SENSE
6
7
8
3
2
1
L1
V IN
1
6
CTRL
2
GND
5
LED
C OUT
SW
L1
3
7
4 CAP
CAP
SW
C OUT
R SENSE
GND
3491 F11
(A) SC70 PACKAGE
(B) DFN PACKAGE
Figure 11. Recommended Component Placement
3491fa
11
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