LTC3541
3541fa
PPLICATIO S I FOR ATIO
U
U
U
PC BOARD LAYOUT CHECKLIST
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3541. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW
trace and the V
IN
trace should be kept short, direct and
wide.
2. Does the LFB pin connect directly to the feedback re-
sistors? The resistive divider R1/R2 must be connected
between the (+) plate of C
OUT
and ground.
3. Does the (+) plate of C
IN
connect to V
IN
as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
4. Keep the switching node, SW, away from the sensitive
LFB node.
5. Keep the () plates of C
IN
and C
OUT
as close as pos-
sible.
DESIGN EXAMPLE
As a design example, assume the LTC3541 is used in
a single lithium-ion battery powered cellular phone ap-
plication. The V
IN
will be operating from a maximum of
4.2V down to about 3V. The load current requirement is
a maximum of 0.5A for the buck output but most of the
time it will be in standby mode, requiring only 2mA. Ef-
fciency at both low and high load currents is important.
The output voltage for the buck is 1.8V. The requirement
for the output voltage of the VLDO is 1.5V while provid-
ing up to 0.3A of current. With this information we can
calculate L using Equation 2:
f I
V
V
V
L
OUT
OUT
IN
=
( ) ?/SPAN>
( )
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
1
1
(2)
Substituting V
OUT
= 1.8V, V
IN
= 3.6V (typ), 擨
L
= 0.2A and
f = 2.25MHz in Equation 3 gives:
V
MHz mA
V
V
H
=
?/DIV>
?/DIV>
?/DIV>
?/DIV>
= ?/DIV>
18
225 200
1
18
36
2
.
. ( )
.
.
(3)
A 2.2礖 inductor works well for this application. For best
efficiency choose a 600mA or greater inductor with less
than 0.2?series resistance.
C
IN
will require an RMS current rating of at least 0.25A =
I
LOAD(MAX)
/2 at temperature . C
OUT
for the buck is chosen
as 22礔 with an ESR of less than 0.2? In most cases, a
ceramic capacitor will satisfy this requirement.
For the feedback resistors of the buck, choose R1 = 80k.
R2 can then be calculated from Equation 4 to be:
V
R k
OUT
2
08
1 1 100
=
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
=
.
(4)
For the feedback resistors of the VLDO, choose R1 = 200k.
R2 can then be calculated from Equation 5 to be:
V
R k
OUT
2
04
1 1 550
=
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
=
.
C
OUT
for the VLDO is chosen as 2.2礔.
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