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LTM4618
APPLICATIONS INFORMATION
EMI Section
The LTM4618 has been evaluated for CISPR22 A and B
Radiated EMI and CISPR25 Conducted EMI. The CISPR25
Conducted EMI test was performed with an input π ?lter
as shown in Figure 17. An RC snubber circuit is optionally
used from the SW pin to the PGND pin to improve the higher
frequency attenuation and EMI limit guard band. Figure 18
shows the CISPR25 conducted emissions plot for 26.5V
input to 3.3V output at 5A load. Several conditions were
evaluated, and Figure 18 results are from the worst-case
condition. The input π ?lter is used to attenuate the re?ected
noise from the regulator input, and is primarily utilized
when the power regulators are closed to the input power
feed to a board, like the input power connectors. If the
regulator design is placed out on the center of the system
board, then the input π ?lter may not be needed because
all of the extra board capacitance and the inductive planes
will provide ?ltering for re?ected emissions. If the system
board has noise sensitive circuitry that is powered from
the same voltage rail as the regulators are, then an input
π ?lter is a good idea to keep regulator noise from cor-
rupting the noise sensitive circuitry on the system board.
Figure 19 shows the CISPR22 B Radiated EMI plots. The
input π ?lter is used to attenuate the re?ected noise from
? Use large PCB copper areas for high current path,
including V IN , PGND and V OUT . It helps to minimize
the PCB conduction loss and thermal stress.
? Test points can be placed on signal pin for monitor-
ing during testing.
? Place high frequency ceramic input and output
capacitors next to the V IN , PGND and V OUT pins to
minimize high frequency noise.
? Place a dedicated power ground layer underneath the
unit.
? To minimize the via conduction loss and reduce mod-
ule thermal stress, use multiple vias for interconnec-
tion between top layer and other power layers.
? Do not put vias directly on the pad, unless they are
capped.
? Use a separated SGND ground copper area for com-
ponents connected to signal pins. Connect the SGND
to PGND underneath the unit.
Figure 20 gives a good example of the recommended
layout.
propagating out onto the input power cables, thus pos-
sibly causing radiated EMI issues. An RC snubber circuit
is optionally used from the SW pin to the PGND pin to
improve the higher frequency attenuation and EMI limit
guard band. A placeholder can accommodate the R SNUB
and C SNUB components with 1.2Ω and 470pF. These
components are probably not necessary, but can be used
V IN
7
6
5
PGND
or adjusted to improve the radiated limit guard bands at
the higher frequencies by attenuating any switch node
ringing due to parasitic values in the high speed switching
paths. It is important to follow the recommended layout
guidelines and use good X5R or X7R ceramic capacitors
CNTRL
4
3
2
1
to get good results.
A
B
C
D
E
F
G
H
J
K
L
M
Layout Checklist/Example
CNTRL
C OUT
The high integration of LTM4618 makes the PC board layout
very simple and easy. However, to optimize its electrical
PGND
C OUT
V OUT
4618 F20
and thermal performance, some layout considerations
are still necessary.
Figure 20. Recommended PCB Layout Example
4618fa
19
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