LM3670_15 Datasheet, PDF (17/25 Pages) Texas Instruments – LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits

LM3670
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SNVS250E – NOVEMBER 2004 – REVISED FEBRUARY 2013
Good layout for the LM3670 can be implemented by following a few simple design rules, as illustrated in
Figure 25.
• Place the LM3670, 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. Place the capacitors and inductor within 0.2 in. (5 mm) of the LM3670.
• 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 LM3670 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 LM3670 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.
• Connect the ground pins of the LM3670, 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 LM3670 by giving it a low-impedance ground connection.
• 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.
• 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 LM3670 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.
• 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.
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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