LM3673TL-1.5EV National Semiconductor, LM3673TL-1.5EV Datasheet - Page 16

• Manufacturer Part Number: LM3673TL-1.5EV
• Description: BOARD EVALUATION LM3673TL-1.5
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3673TL-1.5EV

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.5V
• Current - Output: 350mA
• Voltage - Input: 2.7 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 2MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM3673
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

www.national.com
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 5-Bump
package used for LM3673 has 300 micron solder balls and
requires 10.82 mils 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 7 mil wide, for a sec-
tion approximately 7 mil long or longer, as a thermal relief.
Then each trace should neck up or down to its optimal width.
The important criteria is symmetry. This ensures the solder
bumps on the LM3673 re-flow evenly and that the device sol-
ders level to the board. In particular, special attention must be
paid to the pads for bumps A1 and A3, because V
are typically connected to large copper planes, inadequate
thermal relief can result in late or inadequate re-flow of these
bumps.
The MicroSMD package is optimized for the smallest possible
size in applications with red or infrared opaque cases. Be-
cause the MicroSMD package lacks the plastic encapsulation
characteristic of larger devices, it is vulnerable to light. Back-
side 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 in-
frared range, shining on the package’s exposed die edges.
BOARD LAYOUT CONSIDERATIONS
PC board layout is an important part of DC-DC converter de-
sign. 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, re-
sulting in poor regulation or instability.
Good layout for the LM3673 can be implemented by following
a few simple design rules below. Refer to Figure 9 for top layer
board layout.
1.
Place the LM3673, inductor and filter capacitors close
together and make the traces short. The traces between
these components carry relatively high switching
currents and act as antennas. Following this rule reduces
IN
and GND
16
2.
3.
4.
5.
6.
In mobile phones, for example, a common practice is to place
the DC-DC converter on one corner of the board, arrange the
CMOS digital circuitry around it (since this also generates
noise), and then place sensitive preamplifiers and IF stages
on the diagonally opposing corner. Often, the sensitive cir-
cuitry is shielded with a metal pan and power to it is post-
regulated to reduce conducted noise, using low-dropout
linear regulators.
radiated noise. Special care must be given to place the
input filter capacitor very close to the V
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 LM3673 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 LM3673 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 magnetic
field reversal between the two half-cycles and reduces
radiated noise.
Connect the ground pins of the LM3673 and filter
capacitors together using generous component-side
copper 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 LM3673 by giving it a low-
impedance ground connection.
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.
Route noise sensitive traces, such as the voltage
feedback path, away from noisy traces between the
power components. The voltage feedback trace must
remain close to the LM3673 circuit and should be direct
but should be routed opposite to noisy components. This
reduces EMI radiated onto the DC-DC converter’s own
voltage feedback trace. A good approach is to route the
feedback trace on another layer and to have a ground
plane between the top layer and layer on which the
feedback trace is routed. In the same manner for the
adjustable part it is desired to have the feedback dividers
on the bottom layer.
Place noise sensitive circuitry, such as radio IF blocks,
away from the DC-DC converter, CMOS digital blocks
and other noisy circuitry. Interference with noise-
sensitive circuitry in the system can be reduced through
distance.
IN
and GND pin.