LTC3541-3 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC3541-3' PDF : 20 Pages View PDF
LTC3541-3
APPLICATIO S I FOR ATIO
85°C is approximately 0.25Ω.The RDS(ON) of the N-channel
switch is approximately 0.4Ω. Therefore, power dissipated
by the part is approximately:
PD = (ILOADBUCK)2 • RSW(ON) + (ILOADVLDO) •
(LVIN – LVOUT) = 145mW
For the 3mm × 3mm DFN package, the qJA is 43°C/W.
Thus, the junction temperature of the regulator is:
TJ = 85°C + (0.145)(43) = 91°C
which is well below the maximum junction temperature
of 125°C.
Note that at higher supply voltages, the junction tempera-
ture is lower due to reduced switch resistance RDS(ON).
PC BOARD LAYOUT CHECKLIST
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3541-3. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW
trace and the VIN trace should be kept short, direct and
wide.
2. Does the (+) plate of CIN connect to VIN as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
3. Keep the switching node, SW, away from the sensitive
LFB node.
4. Keep the (–) plates of CIN and COUT as close as
possible.
DESIGN EXAMPLE
As a design example, assume the LTC3541-3 is used in
a single lithium-ion battery powered cellular phone ap-
plication. The VIN 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 regulator is 1.575V while
providing up to 0.3A of current. With this information we
can calculate L using Equation 2:
L
=
(
1
f ) ( ΔIL
)
VOUT
1−
VOUT
VIN
(2)
Substituting VOUT = 1.8V, VIN = 3.6V (typ), ΔIL = 200mA
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.
CIN will require an RMS current rating of at least 0.25A
= ILOAD(MAX)/2 at temperature. COUT for the buck is
chosen to have a value of 22µF and an ESR of less than
0.25Ω. In most cases, a ceramic capacitor will satisfy
this requirement.
COUT for the VLDO regulator is chosen as 2.2µF.
35413fc
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