LT1937
APPLICATIO S I FOR ATIO
D1
2.2nF
LT1937
FB
D2
R2
1k
5k
R1
6.34Ω
C
OUT
SHDN
2V/DIV
V
IN
= 3.6V
THREE LEDs
15mA
50µs/DIV
1937 F11b
D2: MMBT
1937 F11a
(11a) Recommended Soft-Startup Circuit
Figure 11. Recommended Soft-Startup Circuit and Waveforms
Board Layout Consideration
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To maximize efficiency, switch rise and fall times are made
as short as possible. To prevent electromagnetic interfer-
ence (EMI) problems, proper layout of the high frequency
switching path is essential. The voltage signal of the SW
pin has sharp rise and fall edges. Minimize the length and
L
D
C
O
1
2
3
R2
R1
1937 F12a
5
C
IN
GND
4
SHDN
R2
DIMMING
CONTROL
R3
(SOT-23 Package)
Figure 12. Recommended Component Placement
8
U
I
IN
100mA/DIV
FB
100mV/DIV
W
U U
(11b) Soft-Startup Waveforms
area of all traces connected to the SW pin and always use
a ground plane under the switching regulator to minimize
interplane coupling. In addition, the ground connection for
the feedback resistor R1 should be tied directly to the GND
pin and not shared with any other component, ensuring a
clean, noise-free connection. Recommended component
placement is shown in Figure 12.
V
IN
C
O
L
D
1
2
3
6
5
4
C
IN
V
IN
GND
SHDN
R1
1937 F12b
DIMMING
CONTROL
R3
(SC70 Package)
1937f
LINER [ LINEAR TECHNOLOGY ]
