The input filter capacitor supplies current to the PFET switch

of the LM2619 in the first part of each cycle and reduces

voltage ripple imposed on the input power source. The out-

put filter capacitor smoothes out current flow from the induc-

tor to the load, helps maintain a steady output voltage during

transient load changes and reduces output voltage ripple.

These capacitors must be selected with sufficient capaci-

tance and sufficiently low ESR to perform these functions.

The ESR, or equivalent series resistance, of the filter capaci-

tors is a major factor in voltage ripple.

TABLE 2. Suggested Capacitors and Their Suppliers

Model

Type

Vendor

Phone

FAX

C1, C2 (Input or Output Filter Capacitor)

C2012X5ROJ106M

Ceramic

TDK

847-803-6100

847-803-6296

JMK212BJ106MG

Ceramic

Taiyo-Yuden

847-925-0888

847-925-0899

ECJ3YB0J106K

Ceramic

Panasonic

714-373-7366

714-373-7323

JMK325BJ226MM

Ceramic

Taiyo-Yuden

847-925-0888

847-925-0899

C3225X5RIA226M

Ceramic

TDK

847-803-6100

847-803-6296

MICRO SMD PACKAGE ASSEMBLY AND USE

Use of the micro SMD package requires specialized board

layout, precision mounting and careful reflow techniques, as

detailed

in

National

Semiconductor

Application

Note

AN-1112. Refer to the section Surface Mount Technology

(SMT) Assembly Considerations. For best results in assem-

bly, alignment 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 pre-

vents 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 AN-1112

for specific instructions how to do this.

The 10-Bump package used for the LM2619 has 300 micron

solder 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

6–7mil wide, for a section approximately 6mil long, as a

thermal relief. Then each trace should neck up or down to its

optimal width. The important criterion is symmetry. This en-

sures the solder bumps on the LM2619 reflow evenly and

that the device solders level to the board. In particular,

special attention must be paid to the pads for bumps D3–B3.

Because PGND and PVIN are typically connected to large

copper planes, inadequate thermal reliefs can result in late

or inadequate reflow 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

sensitivity.

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 reflow 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 LM2619 can be implemented by follow-

ing a few simple design rules.

1.

Place the LM2619 on 10.82 mil (10.82/1000 in.) pads.

As a thermal relief, connect to each pad witha7mil

wide, approximately 7 mil long traces, and then incre-

mentally increase each trace to its optimal width. The

important criterion is symmetry to ensure the solder

bumps on the LM2619 reflow evenly (see micro SMD

Package Assembly and Use).

2.

Place the LM2619, inductor and filter capacitors close

together and make the traces 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

0.2 in. (5 mm) of the LM2619.

3.

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 LM2619 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 LM2619 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.

4.

Connect the ground pins of the LM2619, and filter ca-

pacitors together using generous component-side cop-

per fill as a pseudo-ground plane. Then, connect this to

the ground-plane (if one is used) with several vias. This

reduces ground-plane noise by preventing the switching

currents from circulating through the ground plane. It

also reduces ground bounce at the LM2619 by giving it

a low-impedance ground connection.

5.

Use wide traces between the power components and for

power connections to the DC-DC converter circuit. This

reduces voltage errors caused by resistive losses across

the traces.

6.

Route noise sensitive traces, such as the voltage feed-

back path, away from noisy traces between the power

components. The voltage feedback trace must remain

close to the LM2619 circuit and should be routed directly

from V

opposite to noise components. This reduces EMI radi-

ated onto the DC-DC converter’s own voltage feedback

trace.

www.national.com

13