TABLE 1. Suggested Inductors And Their Suppliers

Model

Size (WxLxH) [mm]

Vendor

MIPW3226D3R0M

3.2 x 2.6 x 1.0

FDK

1098AS-3R3M

3.0 x 2.8 x 1.2

TOKO

NR3015T3R3M

3.0 x 3.0 x 1.5

Taiyo-Yuden

1098AS-2R7M

3.0 x 2.8 x 1.2

TOKO

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.

TABLE 2. Suggested Capacitors And Their Suppliers

Model

Vendor

C2012X5R0J106M,10µF, 6.3V

TDK

C1608X5R0J475M, 4.7µF, 6.3V

TDK

0805ZD475KA 4.7µF, 10V

AVX

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

down mode.

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

20166308

FIGURE 4. Current Loop

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