LM3203TLX National Semiconductor Corporation, LM3203TLX Datasheet - Page 13

• Manufacturer Part Number: LM3203TLX
• Description: Step-down Dc-dc Converter With Bypass Mode For Cdma / Wcdma Rf Power Amplifiers
• Manufacturer: National Semiconductor Corporation

Application Information
INDUCTOR SELECTION
A 3.3µH inductor with saturation current rating over 940mA is
recommended for almost all applications. The inductor resis-
tance should be less than 0.3Ω for better efficiency. Table 1
lists suggested inductors and suppliers.
A 2.2uH inductor can be used if maximum current in PWM
mode is 300mA. For low-cost applications, an unshielded
bobbin inductor can be used. For noise-critical applications,
a toroidal or shielded-bobbin inductor should be used. A
good practice is to layout the board with footprints accom-
modating both types for design flexibility. This allows substi-
tution of an unshielded inductor, in the event that noise from
low-cost bobbin models is unacceptable. Saturation occurs
when the magnetic flux density from current through the
windings of the inductor exceeds what the inductor’s core
material can support with a corresponding magnetic field.
This can cause poor efficiency, regulation errors or stress to
a DC-DC converter like the LM3203.
CAPACITOR SELECTION
The LM3203 is designed to be used with ceramic capacitors.
Use a 10µF ceramic capacitor for the input and a 4.7µF
ceramic capacitor for the output. Ceramic capacitors such as
X5R, X7R and B are recommended for both filters. These
provide an optimal balance between small size, cost, reliabil-
ity and performance for cell phones and similar applications.
Table 2 lists suggested capacitors and suppliers.
The DC bias characteristics of the capacitor must be consid-
ered when selecting case sizes. If smaller case size such as
1608 (0603) is selected, the dc bias could reduce the cap
value by as much as 40%, in addition to the 20% tolerances
and 15% temperature coefficients. Request dc bias curves
from manufacturer when making selection.The device has
been designed to be stable with output capacitance as low
as 3µF to account for capacitor tolerances.This value in-
cludes dc bias reduction, manufacturing tolerences and
temp coefficients.
The input filter capacitor supplies AC current drawn by the
PFET switch of the LM3203 in the first part of each cycle and
reduces the voltage ripple imposed on the input power
source. A 0.1µF capacitor is also recommended close to V
pin. The output filter capacitor absorbs the AC inductor cur-
rent, helps maintain a steady output voltage during transient
load changes and reduces output voltage ripple. These ca-
pacitors must be selected with sufficient capacitance and
TABLE 2. Suggested Capacitors and Their Suppliers.
TABLE 1. Suggested Inductors and Their Suppliers
4.7µF for C
C3216X5R1A106K
LMK212BJ475MG
C2012X5R0J475K
DO3314-332MX
NR3015T3R3M
Model
Model
OUT
and 10µF for C
Size (WxLxH) [mm]
3.3 x 3.3 x 1.4
3.0 x 3.0 x 1.5
2012 (0805)
2012 (0805)
3216 (1206)
Size (EIA)
IN
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sufficiently low ESR (Equivalent Series Resistance) to per-
form these functions. The ESR of the filter capacitors is
generally a major factor in voltage ripple.
MICRO SMD PACKAGE ASSEMBLY AND USE
Use of the Micro SMD package requires specialized board
layout, precision mounting and careful re-flow techniques, as
detailed in National Semiconductor Application Note 1112.
Refer to the section Surface Mount Technology (SMD) As-
sembly Considerations. For best results in assembly, align-
ment ordinals on the PC board should be used to facilitate
placement of the device. The pad style used with Micro SMD
package must be the NSMD (non-solder mask defined) type.
This means that the solder-mask opening is larger than the
pad size. This prevents a lip that otherwise forms if the
solder-mask and pad overlap, from holding the device off the
surface of the board and interfering with mounting. See
Application Note 1112 for specific instructions how to do this.
The 10-Bump package used for the LM3203 has 300 micron
solder balls and requires 10.82 mil pads for mounting on the
circuit board. The trace to each pad should enter the pad
with a 90˚ entry angle to prevent debris from being caught in
deep corners. Initially, the trace to each pad should be 6-7
mil wide, for a section approximately 6 mil long or longer, as
a thermal relief. Then each trace should neck up or down to
its optimal width. The important criterion is symmetry. This
ensures the solder bumps on the LM3203 re-flow evenly and
that the device solders level to the board. In particular,
special attention must be paid to the pads for bumps B3, C3
and D3. Because PGND and PV
large copper planes, inadequate thermal relief can result in
inadequate re-flow of these bumps.
The Micro SMD package is optimized for the smallest pos-
sible size in applications with red or infrared opaque cases.
Because the Micro SMD package lacks the plastic encapsu-
lation characteristic of larger devices, it is vulnerable to light.
Backside metalization and/or epoxy coating, along with front-
side shading by the printed circuit board, reduce this sensi-
tivity. However, the package has exposed die edges. In
particular, Micro SMD devices are sensitive to light, in the
red and infrared range, shining on the package’s exposed
die edges.
Do not use or power-up the LM3203 while subjecting it to
high intensity red or infrared light; otherwise degraded, un-
predictable or erratic operation may result. Examples of light
sources with high red or infrared content include the sun and
halogen lamps. Place the device in a case opaque to red or
infrared light.
BOARD LAYOUT CONSIDERATIONS
PC board layout is an important part of DC-DC converter
design. Poor board layout can disrupt the performance of a
DC-DC converter and surrounding circuitry by contributing to
EMI, ground bounce, and resistive voltage loss in the traces.
These can send erroneous signals to the DC-DC converter,
resulting in poor regulation or instability. Poor layout can also
result in re-flow problems leading to poor solder joints be-
tween the Micro SMD package and board pads. Poor solder
joints can result in erratic or degraded performance. Good
layout for the LM3203 can be implemented by following a
few simple design rules.
IN
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