LM3202TL datasheet(13/15 Pages) NSC

13 / 15 page

Application Information (Continued)

CAPACITOR SELECTION

The LM3202 is designed for ceramic capacitor for its input

and output filters. Use a 10µF ceramic capacitor for input

and a 4.7µF ceramic capacitor for output. Ceramic capaci-

tors types such as X5R, X7R are recommended to use for

both filters. These provide an optimal balance between small

size, cost, reliability and performance for cell phones and

similar applications. Table 3 lists suggests some part num-

bers and suppliers. DC bias characteristics of the capacitors

must be considered when selecting the voltage rating and

case size of the capacitor. Smaller case sizes for the output

mitigates piezo electric vibrations of the capacitor when the

output voltage is stepped up and down at fast rates however

they have a bigger percentage drop in value with dc bias.

Use of multiple 2.2µF or 1µF capacitors can also be consid-

ered.

TABLE 3. Suggested capacitors and their suppliers

Model

Vendor

JMK212BJ475, 4.7µF, 6.3V

Taiyo-Yuden

GRM188R60J475, 4.7µF, 6.3V

MuRata

C2012X5R0J106,10µF, 6.3V

TDK

The input filter capacitor supplies AC current drawn by the

PFET switch of the LM3202 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

LM3202 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 LM3202 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

LM3202 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 LM3202 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 LM3202 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 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

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

IC, resulting in poor regulation or instability. Poor layout can

also result in re-flow problems leading to poor solder joints

between the Micro SMD package and board pads. Poor

solder joints can result in erratic or degraded performance.

Good layout for the LM3202 can be implemented by follow-

ing a few simple design rules.

Place the LM3202 on 10.82mil pads. As a thermal relief,

connect to each pad with a 7mil wide, approximately

7mil long traces, and when incrementally increase each

trace to its optimal width. The important criterion is sym-

metry to ensure the solder bumps on the LM3202 re-flow

evenly (see Micro SMD Package Assembly and Use).

Place the LM3202, inductor and filter capacitors close

together and make the trace short. The traces between

these components carry relatively high switching cur-

rents and act as antennas. Following this rule reduces

radiated noise. Place the capacitors and inductor within

0.2inch (5mm) of the LM3202.

Arrange the components so that the switching current

loops curl in the same direction. During the first half of

each cycle, current flows from the input filter capacitor,

through the LM3202 and inductor to the output filter

capacitor and back through ground, forming a current

loop. In the second half of each cycle, current is pulled

up from ground, through the LM3202 by the inductor, to

the output filter capacitor and then back through ground,

forming a second current loop. Routing these loops so

the current curls in the same direction prevents mag-

netic field reversal between the two half-cycles and re-

duces radiated noise.

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