LM3208TL/NOPBPB National Semiconductor, LM3208TL/NOPBPB Datasheet - Page 15

• Manufacturer Part Number: LM3208TL/NOPBPB
• Manufacturer: National Semiconductor

Specifications of LM3208TL/NOPBPB

• Lead Free Status / Rohs Status: Supplier Unconfirmed

Application Information
If a smaller inductance inductor is used in the application, the
LM3208 may become unstable during line and load tran-
sients, and V
For low-cost applications, an unshielded bobbin inductor is
suggested. For noise-critical applications, a toroidal or
shielded-bobbin inductor should be used. A good practice is
to lay out the board with footprints accommodating both
types for design flexibility. This allows substitution of a low-
noise toroidal 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 con-
verter like the LM3208.
CAPACITOR SELECTION
The LM3208 is designed for use with ceramic capacitors for
its input and output filters. Use a 10µF ceramic capacitor for
input and a 4.7µF ceramic capacitor for output. They should
maintain at least 50% capacitance at DC bias and tempera-
ture conditions. Ceramic capacitor types such as X5R, X7R
and B are recommended for both filters. Table 2 lists some
suggested part numbers and suppliers. DC bias character-
istics of the capacitors must be considered when selecting
the voltage rating and case size of the capacitor. If it is
necessary to choose a 0603-size capacitor for C
C
evaluated on the system board. Use of multiple 2.2µF or 1µF
capacitors in parallel may also be considered.
The input filter capacitor supplies AC current drawn by the
PFET switch of the LM3208 in the first part of each cycle and
reduces the voltage ripple imposed on the input power
source. The output filter capacitor absorbs the AC inductor
current, helps maintain a steady output voltage during tran-
sient load changes and reduces output voltage ripple. These
capacitors must be selected with sufficient capacitance and
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.
EN PIN CONTROL
Drive the EN pin using the system controller to turn the
LM3208 ON and OFF. Use a comparator, Schmidt trigger or
logic gate to drive the EN pin. Set EN high (
normal operation and low (
down mode.
MIPW3226D3R0M
1098AS-3R3M
NR3015T3R3M
1098AS-2R7M
TABLE 2. Suggested Capacitors And Their Suppliers
OUT
TABLE 1. Suggested Inductors And Their Suppliers
C1608X5R0J475M, 4.7µF, 6.3V
, the operation of the LM3208 should be carefully
C2012X5R0J106M,10µF, 6.3V
0805ZD475KA 4.7µF, 10V
Model
CON
transient response times may be affected.
Model
Size (WxLxH) [mm]
3.2 x 2.6 x 1.0
3.0 x 2.8 x 1.2
3.0 x 3.0 x 1.5
3.0 x 2.8 x 1.2
<
0.5V) for a 0.01µA (typ.) shut-
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1.2V) for
IN
and
15
Set EN low to turn off the LM3208 during power-up and
under voltage conditions when the power supply is less than
the 2.7V minimum operating voltage. The part is out of
regulation when the input voltage is less than 2.7V. The
LM3208 is designed for mobile phones where the system
controller controls operation mode for maximizing battery life
and requirements for small package size outweigh the addi-
tional size required for inclusion of UVLO (Under Voltage
Lock-Out) circuitry.
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 8-Bump package used for LM3208 has 300micron sol-
der balls and requires 10.82mil 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 7mil
wide, for a section approximately 7mil long, 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 LM3208 re-flow evenly and that the
device solders level to the board. In particular, special atten-
tion must be paid to the pads for bumps A1 and A3. Because
PGND and PV
planes, inadequate thermal relief’s can result in late or inad-
equate 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 metallization and/or epoxy coating, along with
front-side shading by the printed circuit board, reduce this
sensitivity. 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.
BOARD LAYOUT CONSIDERATIONS
IN
FIGURE 4. Current Loop
are typically connected to large copper
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