LM3674_15 Datasheet, PDF (17/26 Pages) Texas Instruments – LM3674 2-MHz, 600-mA Step-Down DC-DC Converter in SOT-23

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8.2.2.3 Application Curves
LM3674
SNVS405G – DECEMBER 2005 – REVISED APRIL 2015
Table 5. Related Plots
PLOT TITLE
Output Voltage vs Supply Voltage
Output Voltage vs Temperature
Output Voltage vs Output Current
Efficiency vs Output Current
Efficiency vs Output Current
Efficiency vs Output Current
Efficiency vs Output Current
Line Transient Response
Load Transient
Start-Up
Start-Up
FIGURE
Figure 4
Figure 5
Figure 6
Figure 9
Figure 10
Figure 11
Figure 12
Figure 14
Figure 15
Figure 16
Figure 17
9 Power Supply Recommendations
The LM3674 requires a single supply input voltage. This voltage can range between 2.7 V to 5.5 V and be able
to supply enough current for a given application.
10 Layout
10.1 Layout Guidelines
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 device, resulting in poor regulation or
instability.
Good layout for the LM3674 can be implemented by following a few simple design rules, as illustrated in
Figure 21.
1. Place the LM3674, 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 by 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 LM3674 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 LM3674 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 LM3674, 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 LM3674 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 LM3674 circuit and should be direct but
should be routed opposite to noisy components. This reduces the EMI radiated onto the voltage feedback
trace of the DC-DC converter. 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
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