Datasheet

LM3370
SNVS406N –NOVEMBER 2005–REVISED MAY 2013 www.ti.com
Spread Spectrum (SS)
The LM3370 features Spread Spectrum capability, via I
2
C, to reduce the noise amplitude of the switching
frequency during data transmission. The feature can be enabled by activating the appropriate control register bit
(see Table 2 for details). The main clock of the LM3370 features spread spectrum at F
OSC
= 2 MHz ± 22 kHz (
peak frequency deviation) with the modulation frequency of either 1 kHz (default) or 2 kHz via I
2
C. This help
reduce noise caused by the harmonics present in the waveforms at the switch pins of the buck regulators. It is
controlled by I
2
C in the following manner:
I
2
C bit Modulation Frequency
SS_fmod = 1 (default) 1 kHz
SS_fmod = 0 2 kHz
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 LM3370 can be implemented by following a few simple design rules:
1. Place the LM3370, 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. (5mm) of the LM3370.
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 LM3370 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 LM3370 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 LM3370, 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 LM3370 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 LM3370 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.
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.
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.
DSBGA Package Assembly and Use
Use of the DSBGA package requires specialized board layout, precision mounting and careful re-flow
techniques, as detailed in Texas Instruments Application Note 1112 (SNVA009). Refer to the section Surface
Mount Technology Assembly Considerations. For best results in assembly, alignment ordinals on the PC board
should be used to facilitate placement of the device. The pad style used with DSBGA package must be the
NSMD (non-solder mask defined) type. This means that the solder-mask opening is larger than the pad size.
This prevents a lip that otherwise forms if the solder-mask and pad overlap, from holding the device off the
surface of the board and interfering with mounting. See Application Note 1112 (SNVA009) for specific
instructions how to do this. The 20-bump DSBGA package used for the LM3370 has 300 micron solder balls and
requires 10.82 mils pads for mounting on the circuit board. The trace to each pad should enter the pad with a 90°
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