LM3670MF-1.2EV National Semiconductor, LM3670MF-1.2EV Datasheet - Page 15

• Manufacturer Part Number: LM3670MF-1.2EV
• Description: BOARD EVALUATION LM3670MF-1.2
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3670MF-1.2EV

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.2V
• Current - Output: 350mA
• Voltage - Input: 2.5 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 1MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM3670
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -
• Lead Free Status / Rohs Status: Not Compliant

Application Information
BOARD LAYOUT CONSIDERATIONS
PC board layout is an important part of DC-DC converter
design. Poor board layout can disrupt the performance of a
Good layout for the LM3670 can be implemented by follow-
ing a few simple design rules, as illustrated in .
1.
2. Arrange the components so that the switching current
3. Connect the ground pins of the LM3670, and filter ca-
together and make the traces short. The traces between
these components carry relatively high switching cur-
rents and act as antennas. Following this rule reduces
radiated noise. Place the capacitors and inductor within
0.2 in. (5 mm) of the LM3670.
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 mag-
netic field reversal between the two half-cycles and re-
duces radiated noise.
pacitors together using generous component-side cop-
per fill as a pseudo-ground plane. Then, connect this to
Place the LM3670, inductor and filter capacitors close
FIGURE 6. Board Layout Design Rules for the LM3670
(Continued)
15
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.
4. Use wide traces between the power components and for
5. Route noise sensitive traces, such as the voltage feed-
6. Place noise sensitive circuitry, such as radio IF blocks,
In mobile phones, for example, a common practice is to
place the DC-DC converter on one corner of the board,
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.
power connections to the DC-DC converter circuit. This
reduces voltage errors caused by resistive losses across
the traces.
back 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.
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.
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