LM3205TLX datasheet(14/18 Pages) NSC

14 / 18 page

Application Information (Continued)

The input filter capacitor supplies AC current drawn by the

PFET switch of the LM3205 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

LM3205 ON and OFF. Use a comparator, Schmidt trigger or

logic gate to drive the EN pin. Set EN high (>1.2V) for

normal operation and low (<0.5V) for a 0.01µA (typ.) shut-

down mode.

Set EN low to turn off the LM3205 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

LM3205 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.

microSMD PACKAGE ASSEMBLY AND USE

Use of the microSMD 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 microSMD

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 LM3205 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 LM3205 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, A3 and B3.

Because PGND and PVIN are typically connected to large

copper planes, inadequate thermal relief’s can result in late

or inadequate re-flow of these bumps.

The microSMD package is optimized for the smallest pos-

sible size in applications with red or infrared opaque cases.

Because the microSMD 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, microSMD devices are sensitive to light, in the red

and infrared range, shining on the package’s exposed die

edges.

LLP PACKAGE ASSEMBLY AND USE

Use of the LLP package requires specialized board layout,

precision mounting and careful re-flow techniques, as de-

tailed in National Semiconductor Application Note 1187. Re-

fer to the section Surface Mount Technology (SMT) Assem-

bly Recommendations. For best results in assembly,

alignment ordinals on the PC board should be used to

facilitate placement of the device and must attach to the

DAP(Die Attach Pad) of the LLP package. The pad style

used with LLP package must be the NSMD (non-solder

mask defined) type. This means that the solder-mask open-

ing 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 1187 for specific in-

structions how to do this.

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