LTC3541-1 Datasheet, PDF (16/20 Pages) Linear Technology – High Efficiency Buck + VLDO Regulator

LTC3541-1
APPLICATIO S I FOR ATIO
PC BOARD LAYOUT CHECKLIST
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3541-1. 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 LTC3541-1 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-
ficiency 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:
L
=
(
f
1
)(ΔI_L
)
V_OUT


1−
V_OUT
V_IN


(2)
Substituting V_OUT = 1.8V, V_IN = 3.6V (typ), ΔI_L = 0.2A and
f = 2.25MHz in Equation 3 gives:
L
=
1.8V
2.25MHz(200mA)
_1−
1.8V
3.6V


=
2µH
(3)
A 2.2µH inductor works well for this application. For best
efficiency choose a 600mA or greater inductor with less
than 0.2Ω 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 22µF with an ESR of less than 0.2Ω. 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:
R2 =


V_OUT
0.8
− 1^{}
R1= 100k
(4)
For the feedback resistors of the VLDO, choose R1 = 200k.
R2 can then be calculated from Equation 5 to be:
R2 =


V_OUT
0.4
− 1^{}
R1=
550k
C_OUT for the VLDO is chosen as 2.2µF.
35411fa
16