Application Note
22 October 2008
PDF Name: pwb_layout_cons.pdf
Application Guidelines for Non-Isolated Converters
AN04-006: PWB Layout Considerations
Introduction
Non-Isolated POL dc-dc converters are switching buck
regulators which require careful layout considerations
when designing on to a printed wiring board (PWB).
Many applications using these non-isolated dc-dc
converters utilize high-density multi-layer circuit boards,
and proper component placement and power/control
routing is critical for trouble-free operation of the power
modules. This application note provides generic
guidelines for laying out the Austin Lynx and TLynx
series of non-isolated dc-dc modules. Please consult
Lineage Power Technical Representatives for guidelines
in more specialized applications.
0.1µF (0603) and 1.0µF (0603) small package ceramic
capacitors should be placed at input and output of the
module in the above order. For a more detailed
discussion of input filtering for the Austin Lynx series,
please refer to
Application Note AN04-002
titled
“Application Guidelines for Non-Isolated Converters:
Input Filtering Considerations”.
Figure 2 shows an example layout for a Austin Lynx
Series module. This example details the key guidelines
to be followed when designing the module on to your
board. For simplicity, all three power traces (input, output
and ground) are assumed to be on the top layer of the
PWB – where the Austin Lynx module is placed. The first
key guideline is to extend the ground plane to the area
underneath the module. It is not recommended that this
space be utilized for routing signal traces unless they are
in inner layers underneath the ground plane. The V
OUT
and ground planes are placed close together to minimize
interconnect inductance on the output side. Output
capacitors (C
OUT
) are connected as close to the
General Guidelines
Location of the Module
Dc-dc converters, because of their switching action, are
a source of rapidly varying electrical and magnetic fields.
The EMI spectrum from these modules can range from
the switching frequency (typically around 300 kHz for the
Austin Lynx series modules) to harmonics in the MHz
range. To minimize effects on other components on the
PWB, location of the dc-dc converter module should be
carefully considered. Proper input and output filtering can
reduce the noise levels at the terminals of the modules.
Suggested layout of the traces used to connect the
modules and locations of external components are
provided later in this application note. These, along with
good analog design layout practices are sufficient to
achieve proper performance when using these modules.
Minimizing Loop Area
The input current of a buck converter is discontinuous,
and while both the Austin Lynx and Tlynx series of
modules have input filter capacitors incorporated in the
module, the current into the module does have a
significant ripple component, which leads to a voltage
ripple being superimposed on the input source. This high
frequency ripple voltage and current could be a potential
source of noise. To avoid noise coupling, it is
recommended that the loop area for both power and
signal traces to the dc-dc module be minimized. In
addition, input and output capacitor, located as close as
possible to the module, are recommended for high
frequency filtering. Figure 1 shows a typical application
circuit of the Austin Lynx module, incorporating these
recommendations. Input (C
IN
) and output (C
OUT
) are
ceramic Low ESL and ESR capacitors. Such capacitors
are available from companies such as Syfer, TDK and
Murata.
Capacitors used for minimizing the ripple
component are termed as bulk capacitors with
capacitance needed being in the order of tens or
hundreds of microfarads. For reducing high-frequency
switching noise at the input and output of the module,
Fig. 1. Typical application circuit of an Austin Lynx series
module.
Fig. 2. Simplified layout for the Austin Lynx and SuperLynx
series modules.
LINEAGE
POWER
1
LINEAGEPOWER [ LINEAGE POWER CORPORATION ]
