For power converters, a hot
loop PCB layout with minimum
parasitic parameters can
improve the power efficiency,
lower the voltage ringing, and
reduce EMI.
"
"
Hot Loop PCB ESR and ESL vs.
Decoupling Capacitor Position
In this section, the impacts of C
IN
position are
investigated based on Analog Devices' DC2665A-B
demo board for the LTM4638 µModule
®
regulator. The
LTM4638 is an integrated 20V
IN
, 15A step-down buck
converter module in a tiny 6.25mm × 6.25mm × 5.02mm
BGA package. It offers high power density, fast
transient response, and high efficiency. The module
integrates a small HF ceramic C
IN
inside, though it is not
sufficient yet, limited by the module package size.
Figures 2 through 4 illustrate three different hot loops
on the demo board with additional external C
IN
. The first
one is the vertical hot loop 1 (Figure 2), where C
IN1
is
placed on the bottom layer just beneath the μModule
regulator. The µModule V
IN
and GND BGA pins are
connected to C
IN1
directly through the vias. These
connections provide the shortest hot loop path on the
demo board.
The second hot loop is the vertical hot loop 2 (Figure 3),
where C
IN2
is still placed on the bottom layer but moved
to the side area of the μModule regulator. As a result,
an extra PCB trace is added to the hot loop, and larger
ESL and ESR are expected compared to vertical hot
loop 1.
The third hot loop option is the horizontal hot loop
(Figure 4), where C
IN3
is placed on the top layer close to
the μModule regulator. The µModule V
IN
and GND pins
are connected to C
IN3
through the top layer of copper
without going through vias. Nevertheless, the V
IN
copper width on the top layer is limited by the other
pinout, resulting in an increased loop impedance
compared to that of vertical hot loop 1.
Figure 2: Vertical hot loop 1: (a) top view and (b) side view.
(Source: Analog Devices)
Figure 3: Vertical hot loop 2: (a) top view and (b) side view.
(Source: Analog Devices)
Figure 4: Horizontal hot loop: (a) top view and (b) side view.
(Source: Analog Devices)
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