Datasheet

11
LT3491
3491fa
5
6
7
8
4
3
2
1
L1
C
IN
C
OUT
GND
GND
V
IN
CTRL
LED
CAP
R
SENSE
SW
(A) SC70 PACKAGE
L1
C
IN
C
OUT
3491 F11
V
IN
CTRL
LED
CAP
R
SENSE
SW
(B) DFN PACKAGE
4
5
GND
7
6
3
2
1
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
powered off a 3.3V logic supply voltage.
APPLICATIO S I FOR ATIO
WUUU
Figure 10. 2 AA Cells to Three White LEDs
Figure 11. Recommended Component Placement
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.
CTRL
V
IN
C2
2.2µF
C1: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN GMK325BJ225ML
L1: MURATA LQH32CN100
C1
1µF
L1
10µH
C1
0.1µF
2 AA CELLS
2V TO 3.2V
3.3V
R
SENSE
10Ω
3491 F10
LT3491
SHUTDOWN AND
DIMMING CONTROL
SW
CAP
LED
GND