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PC Board Layout
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PC Board Layout
The LM3102 regulation, over-voltage, and current limit comparators are very fast so they will respond to
short duration noise pulses. Layout is therefore critical for optimum performance. It must be as neat and
compact as possible, and all external components must be as close to their associated pins of the
LM3102 as possible. The loop formed by the input capacitors (C1 and C2), the main and synchronous
MOSFET internal to the LM3102, and the PGND pin should be as small as possible. The connection from
the PGND pin to the input capacitors should be as short and direct as possible. Vias should be added to
connect the ground of the input capacitors to a ground plane, located as close to the capacitor as
possible. The bootstrap capacitor C4 should be connected as close to the SW and BST pins as possible,
and the connecting traces should be thick. The feedback resistors and capacitor R3, R4, and C9 should
be close to the FB pin. A long trace running from V
OUT
to R3 is generally acceptable since this is a low
impedance node. Ground R4 directly to the AGND pin (pin 7). The output capacitor C10, C11 should be
connected close to the load and tied directly to the ground plane. The inductor L1 should be connected
close to the SW pin with as short a trace as possible to reduce the potential for EMI (electromagnetic
interference) generation. If it is expected that the internal dissipation of the LM3102 will produce excessive
junction temperature during normal operation, making good use of the PC board’s ground plane can help
considerably to dissipate heat. The exposed pad on the bottom of the LM3102 IC package can be
soldered to the ground plane, which should extend out from beneath the LM3102 to help dissipate heat.
The exposed pad is internally connected to the LM3102 IC substrate. Additionally the use of thick traces,
where possible, can help conduct heat away from the LM3102. Using numerous vias to connect the die
attached pad to the ground plane is a good practice. Judicious positioning of the PC board within the end
product, along with the use of any available air flow (forced or natural convection) can help reduce the
junction temperature.
Figure 3. LM3102 Demonstration Board PCB Top Overlay
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SNVA248 – October 2007
AN-1646 LM3102 Demonstration Board Reference Design
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