lm8801tmx National Semiconductor Corporation, lm8801tmx Datasheet - Page 12

• Manufacturer Part Number: lm8801tmx
• Description: High Precision 6mhz, 600 Ma Synchronous Step-down Dc-dc Converter For Mobile Applications
• Manufacturer: National Semiconductor Corporation

www.national.com
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 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 1112 for
specific instructions how to do this.
1.
2.
3.
Place the LM8801 on 8.26 mil pads. As a thermal relief,
connect each pad with a 7 mil wide, approximately 7 mil
long trace, and then incrementally increase each trace to
its optimal width. The important criterion is symmetry to
ensure the solder bumps re-flow evenly (see AN-1112,
Micro SMD Package Assembly and Use).
Place the LM8801, 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
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 LM8801 and inductor to the output filter
CL05A475MQ5NRNC (C
GRM155R60J225ME15 (C
JMK105BJ225MV-F (C
Model
IN
or C
IN
)
IN
TABLE 2. Suggested Capacitors and Suppliers
OUT
)
FIGURE 4. LM8801 Board Layout (Top View)
IN
)
and GND pin.
12
Taiyu Yuden
Samsung
Vendor
Muruta
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. Poor layout can also
result in re-flow problems leading to poor solder joints be-
tween the micro SMD package and board pads. Poor solder
joints can result in erratic or degraded performance.
Good layout for the LM8801 can be implemented by following
a few simple design rules, as illustrated in Figure 4.
4.
capacitor and back through ground, forming a current
loop. In the second half of each cycle, current is pulled
up from ground, through the LM8801 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 LM8801, 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 LM8801 by giving it a low-
impedance ground connection.
0402 (1005)
0402 (1005)
0402 (1005)
Inch (mm)
Case Size
30082209