LM3678_15 Datasheet, PDF (12/19 Pages) Texas Instruments – High-Performance Miniature 1.5-A Step-Down DC-DC Converter for Handheld Applications

LM3678
SNVS464C – APRIL 2008 – REVISED MAY 2013
Table 3. Suggested Capacitors and Their Suppliers
Model
10µF for C_IN
GRM21BR60J106K
JMK212BJ106K
C2012X5R0J106K
22µF for C_OUT
JMK212BJ226MG
Type
Ceramic, X5R
Ceramic, X5R
Ceramic, X5R
Ceramic, X5R
Vendor
Murata
Taiyo-Yuden
TDK
Taiyo-Yuden
Voltage Rating
6.3V
6.3V
6.3V
6.3V
www.ti.com
Case Size
Inch (mm)
0805 (2012)
0805 (2012)
0805 (2012)
0805 (2012)
Board Layout Considerations
PC board layout is an important part of DC-DC converter design. 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, resulting in poor regulation or
instability.
Good layout for the LM3678 can be implemented by following a few simple design rules below.
1. Place the LM3678, 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_IN
and GND pin.
2. 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 LM3678 and inductor to the output filter
capacitor and back through ground, forming a current loop. In the second half of each cycle, current is pulled
up from ground through the LM3678 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.
3. Connect the ground pins of the LM3678 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 LM3678 by giving it a low-impedance ground connection.
4. Use wide traces between the power components and for power connections to the DC-DC converter circuit.
This reduces voltage errors caused by resistive losses across the traces.
5. Route noise sensitive traces, such as the voltage feedback path, away from noisy traces between the power
components. The voltage feedback trace must remain close to the LM3678 circuit and should be direct but
should be routed opposite to noisy components. This reduces EMI radiated onto the DC-DC converter’s own
voltage feedback trace. A good approach is to route the feedback trace on another layer and to have a
ground plane between the top layer and layer on which the feedback trace is routed. In the same manner for
the adjustable part it is desired to have the feedback dividers on the bottom layer.
6. Place noise sensitive circuitry, such as radio IF blocks, away from the DC-DC converter, CMOS digital blocks
and other noisy circuitry. Interference with noise-sensitive circuitry in the system can be reduced through
distance.
For detailed layout information, refer to Application Note 1722 LM3678 Evaluation Board SNVA289.
In mobile phones, for example, a common practice is to place the DC-DC converter on one corner of the board,
arrange the CMOS digital circuitry around it (since this also generates noise), and then place sensitive
preamplifiers and IF stages on the diagonally opposing corner. Often, the sensitive circuitry is shielded with a
metal pan and power to it is post-regulated to reduce conducted noise, using low-dropout linear regulators.
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