LTC3405 Linear Technology, LTC3405 Datasheet - Page 12

• Manufacturer Part Number: LTC3405
• Description: Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
LTC3405
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3405. These items are also illustrated graphically in
Figures 9 and 10. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW
2. Does the V
3. Does the (+) plate of C
12
trace and the V
wide.
resistors? The resistive divider R1/R2 must be con-
nected between the (+) plate of C
possible? This capacitor provides the AC current to the
internal power MOSFETs.
FB
IN
pin connect directly to the feedback
U
trace should be kept short, direct and
U
IN
connect to V
*ADD R3 WHEN USING CERAMIC C
W
V
OUT
OUT
V
OUT
BOLD LINES INDICATE HIGH CURRENT PATHS
*ADD R3 FOR APPLICATIONS USING A CERAMIC C
SW
L1
and ground.
–
+
IN
Figure 10. LTC3405 Suggested Layout
Figure 9. LTC3405 Layout Diagram
as closely as
C
OUT
PIN 1
L1
U
VIA TO SW NODE
C
OUT
OUT
1
2
3
R3*
LTC3405
RUN
SW
GND
VIA TO V
GND
LTC3405
R3*
MODE
4. Keep the switching node, SW, away from the sensitive
Design Example
As a design example, assume the LTC3405 is used in a
single lithium-ion battery-powered cellular phone
application. The V
4.2V down to about 2.7V. The load current requirement
is a maximum of 0.25A but most of the time it will be in
standby mode, requiring only 2mA. Efficiency at both low
and high load currents is important. Output voltage is
2.5V. With this information we can calculate L using
equation (1),
V
V
C
IN
FB
IN
IN
C
V
V
L
IN
6
5
4
FB
FB
node.
f
C
R1
R2
OUT
FWD
1
R2
I
L
VIA TO GND
V
OUT
C
IN
3405 F09
FWD
will be operating from a maximum of
1
R1
V
V
+
–
IN
IN
3405 F10
V
V
OUT
IN
VIA TO V
OUT
sn3405 3405fs
(3)